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VOLUTION 


R   R   E   L  L 


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Evolution 
and  Genetics 


THE 

MODERN 

THEORY 

OF 

EVOLUTION 


hvt 


David  J.  Merrell 

University  of  Minnesota 

With  illustrations  by 

OLIVIA  JENSEN  INGERSOLL 


HOLT,  RINEHART  AND  WINSTON 
New  York 


Copyright  ©  1962  by  Holt,  Rinehart  and  Winston,  Inc. 
All  Rights  Reserved 

90123    4    9876 


Library  of  Congress  Catalog  Card  Number:  62-8420 

03-010950-7 

Printed  in  the  United  States  of  America 


Preface 


The  human  species,  in  a  remarkable  manner,  has  be- 
come the  dominant  species  on  the  earth.  Man's  range  has  ex- 
panded explosively  out  of  the  tropics,  and  as  he  has  gained 
mastery  over  his  competitors,  his  parasites,  and  his  environ- 
ment, human  numbers  have  increased  at  an  accelerating  pace. 
These  biological  facts  are  of  primary  significance  in  the  world 
of  today.  To  understand  them  requires  an  understanding  of  the 
evolutionary  forces  that  have  been  at  work  in  the  past  and  con- 
tinue to  work  at  present. 

The  study  of  evolution  received  its  major  impetus  just 
over  a  century  ago  with  the  publication  of  Darwin's  Origin  of 
Species  in  1859.  Since  then,  great  progress  has  been  made  in 
biology.  Knowledge  has  accumulated  so  rapidly  that  the  field 
has  splintered  into  a  number  of  subdisciplines.  As  specializa- 
tion has  increased,  the  need  for  a  unifying  principle  in  biology 
has  grown.  The  most  notable  success  in  tying  together  the  many 


VI  •  PREFACE 

threads  of  biological  thought  has  been  achieved  by  a  return  to  the  study  of 
evolution.  It  is  a  return  because,  after  the  original  impetus  from  Darwin's 
work  had  dwindled,  a  rather  strong  reaction  against  Darwinism  developed 
early  in  this  century.  The  validity  of  the  theory  of  evolution  was  generally 
accepted  by  biologists,  but  the  discoveries  of  the  early  geneticists  seemed  to 
cast  considerable  doubt  on  his  theory  of  natural  selection. 

A  reflection  of  the  appraisal  of  Darwin  at  that  time  can  be  found  in 
Nordenskiold's  History  of  Biology  (1927).  "To  raise  the  theory  of  selection, 
as  has  often  been  done,  to  the  rank  of  a  'natural  law'  comparable  in  value  with 
the  law  of  gravity  established  by  Newton  is,  of  course,  quite  irrational,  as  time 
has  already  shown:  Darwin's  theory  of  the  origin  of  species  was  long  ago 
abandoned.  Other  facts  established  by  Darwin  are  all  of  second-rate  value.  But 
if  we  measure  him  by  his  influence  on  the  general  cultural  development  of 
humanity,  then  the  proximity  of  his  grave  to  Newton's  is  fully  justified."  How- 
ever, further  progress,  particularly  in  the  fields  of  genetics,  systematics,  and 
paleontology,  has  led  to  an  increased  understanding  not  only  of  evolution  but 
of  the  mechanism  by  which  it  takes  place. 

The  expanded  theory  of  evolution  that  has  been  recently  developed  is 
sometimes  known  as  neo-Darwinism,  or  as  "the  modern  synthesis,"  and  the 
theory  of  natural  selection  has  proven  to  be  more  resilient  than  Darwin's  critics 
supposed.  Furthermore,  the  study  of  evolution  has  come  to  be  a  unifying  force 
in  biology,  drawing  together  information  from  many  disciplines  into  a  com- 
prehensive and  comprehensible  whole.  At  the  present  time,  more  research  on 
evolutionary  problems  is  being  conducted  than  at  any  time  since  1859. 

It  has  become  routine  for  biologists  to  preface  their  remarks  about 
evolution  with  the  statement,  "Everyone  now  accepts  the  fact  of  evolution." 
However,  my  experience  has  been,  in  talking  with  a  variety  of  audiences,  that 
not  everyone  does  accept  evolution  as  a  fact,  even  though  they  may  have  been 
exposed  to  the  concept.  Furthermore,  since  there  are  millions  of  people  in  many 
parts  of  the  world  who  have  never  even  heard  of  the  theory  of  evolution,  the 
supposition  that  everyone  accepts  it  obviously  needs  some  qualification.  Because 
all  mankind  is  caught  up  in  the  same  evolutionary  skein,  it  seems  highly  de- 
sirable that  all  of  us  should  be  made  aware  of  this  fact.  If  man  continues  to 
pursue  his  evolutionary  future  blindly,  without  awareness  or  regard  for  the 
forces  at  work,  that  future  may  be  bedeviled  by  unnecessary  hazards  and  hard- 
ships. 

Doubts  about  the  validity  of  evolution  by  intelligent  and  supposedly 
educated  people  are  due  in  many  cases  to  the  fact  that  they  have  never  really 
heard  the  evidence  in  its  favor.  The  extent  of  this  ignorance  was  most  forcibly 
impressed  on  me  during  a  recent  talk  with  a  group  of  high  school  biology 
teachers.  As  the  discussion  progressed,  it  became  clear  that  at  least  half  of  this 
select  group  of  teachers  did  not  themselves  believe  in  evolution,  and  thus  it 
was  a  real  problem  for  them  to  decide  how  to  handle  the  subject  in  class.  Their 


PREFACE  •  Vll 

reactions  made  clear  the  need  to  continue  presenting  the  case  for  evolution  to 
new  generations  of  students.  For  unless  students,  at  some  stage  in  their  training, 
are  given  the  opportunity  to  become  acquainted  with  the  nature,  variety,  and 
weight  of  the  data  concerning  evolution,  they  are  required  to  accept  evolution 
on  faith.  It  is,  therefore,  perhaps  not  surprising  that  people  never  exposed  to 
the  evidence  may  find  other  explanations  more  emotionally  satisfying.  Only 
after  the  facts  have  been  reviewed  and  understood  does  the  theory  of  evolution 
become  inescapable.  I  have  no  particular  desire  to  convert  anyone  to  a  belief 
in  evolution,  but  at  the  same  time  I  feel  that  even  those  who  are  unconvinced 
about  evolution  should  be  familiar  with  the  evidence.  At  least  then  their  beliefs 
will  not  be  based  on  ignorance,  and  they  will  know  exactly  what  it  is  that  they 
do  not  believe. 

For  these  reasons  the  first  part  of  the  book  has  been  devoted  to  a 
consideration  of  the  nature  of  the  evidence  for  evolution.  No  more  than  a 
sampling  of  the  wealth  of  material,  of  course,  can  be  presented.  It  is  hoped 
that  for  the  great  majority  of  readers  this  presentation  will  be  sufficiently  con- 
vincing. For  those  who  remain  in  doubt  as  to  the  reality  of  evolution,  the 
references  open  a  number  of  paths  from  which  any  who  follow  them  with  an 
open  mind  can  scarcely  return  unconvinced  that  evolution  has  occurred.  The 
supplementary  reading  suggested  at  the  end  of  each  chapter  can  thus  serve  the 
dual  purpose  of  documenting  statements  in  the  text  and  also  of  giving  addi- 
tional information  to  the  student  desirous  of  learning  more  about  a  particular 
topic.  I  have  referred  freely  to  the  writings  and  opinions  of  many  authors 
without  citing  or  documenting  the  actual  sources  in  the  text.  Since  literature 
citations  and  footnotes  can  be  a  major  source  of  distraction  to  the  weak-willed 
reader,  it  seemed  desirable  to  keep  such  diversions  to  a  minimum  so  that  the 
reader  will  be  better  able  to  follow  the  argument  being  presented.  To  those  not 
cited,  and  thereby  slighted,  my  apologies.  The  references,  though  not  complete, 
should  be  a  sufficient  guide  into  any  area  in  which  further  information  or 
documentation  is  desired. 

This  book  has  been  written  primarily  for  those  who  wish  to  know 
more  about  the  theory  of  evolution  and  the  operation  of  evolutionary  forces. 
The  problem  of  discussing  evolution  is  complicated  by  the  fact  that  it  must 
be  taken  up  a  piece  at  a  time  and  fitted  together  like  a  jig-saw  puzzle.  Only 
when  all  of  the  pieces  are  together  can  the  whole  picture  be  fully  appreciated. 
The  discussion  ranges  over  a  wide  variety  of  subjects,  but  an  effort  has  been 
made  to  develop  each  topic  in  such  a  way  that  the  reader  can  follow  the  argu- 
ment with  only  a  minimum  of  background.  One  of  the  major  hurdles  for  the 
student  of  biology  is  the  number  of  new  terms  that  constantly  appear.  If  he 
does  not  learn  the  vocabulary  so  that  he  can  handle  the  biologists'  jargon,  he 
remains  biologically  illiterate.  As  an  aid  over  this  hurdle,  terms  are  generally 
explained  when  first  introduced,  but  a  glossary  is  also  included  at  the  end  for 
quick  reference. 


Vlll  •  PREFACE 

The  modern  theory  of  the  mechanism  of  evolution  is  a  genetic  theory. 
Since  without  some  understanding  of  genetics  the  modern  theory  of  evolution  is 
incomprehensible,  it  is  essential  to  devote  a  section  of  the  book  to  the  funda- 
mental principles  of  genetics.  From  the  text  the  reader  should  be  able  to  gain 
an  understanding  of  the  basic  genetic  principles,  but  if  he  becomes  interested 
in  pursuing  the  subject  further,  he  should  refer  to  the  numerous  excellent  books 
in  the  field.  Evolution  is  a  population  phenomenon  and  is  best  understood  in 
terms  of  the  genetics  of  populations.  Population  genetics  requires  the  use  of 
some  mathematics,  which,  unfortunately,  causes  consternation  for  some  stu- 
dents. However,  only  rather  simple  examples  have  been  included,  requiring  at 
most  a  knowledge  of  elementary  algebra.  A  dash  of  common  sense  and  a  little 
persistence  in  dealing  with  this  material  will  be  well  rewarded  in  terms  of  the 
insight  gained. 

A  biological  approach  has  been  used  throughout  the  book,  and  no 
attempt  has  been  made  to  explore  the  philosophical  or  religious  implications  of 
the  theory  of  evolution.  This  approach  is  sometimes  disturbing  to  students. 
However,  just  among  the  various  Christian  denominations,  attitudes  range  from 
unqualified  acceptance  to  complete  rejection  of  evolution.  Because  of  the  diver- 
sity of  opinion  and  belief,  generalizations  are  virtually  meaningless,  and  it  seems 
wisest  to  encourage  each  student  to  reconcile  his  knowledge  of  evolution  with 
his  personal  beliefs,  if  this  is  necessary. 

I  wish  to  acknowledge  the  inspiration  of  Dr.  Dwight  E.  Minnich,  who 
first  encouraged  me  to  undertake  teaching  a  course  in  evolution,  and  of  the 
many  students  whose  interest  has  made  this  particular  course  such  a  pleasure  to 
teach.  The  comments  and  suggestions  of  my  colleagues  at  the  University  of 
Minnesota,  James  C.  Underhill,  Joseph  G.  Gall,  John  W.  Hall,  and  Frank  G. 
Nordlie,  have  been  most  helpful,  but  I,  of  course,  am  solely  responsible  for 
the  final  form  of  the  book.  In  a  work  of  this  sort,  covering  as  it  does  subjects 
ranging  from  the  origin  of  life  to  cultural  anthropology,  choices  must  be  made 
in  matters  of  emphasis  and  interpretation.  It  is  hoped  that  the  net  result  is  a 
reasonably  balanced  account  of  current  thought  on  evolution. 

My  collaboration  with  Mrs.  Olivia  Jensen  Ingersoll,  whose  imaginative 
drawings  illustrate  the  book,  of  necessity  was  carried  on  at  long  range  since 
her  home  is  in  Ohio.  However,  her  competence,  both  as  an  illustrator  and  as 
a  zoologist,  greatly  eased  the  problems  involved.  Finally,  I  wish  to  acknowledge 
the  devoted  assistance  of  my  wife,  Jessie,  who  assumed  the  onerous  task  of 
typing  the  manuscript. 

D.  J.  M. 
Minneapolis,  Minnesota 
January,  1962 


Credits 


The  following  illustrations  are  used  with  the  kind  permission  of  the 
authors  and  publishers  listed  below. 

Fig.     1-2.     Cott,  H.  B.,  1940,  Adaptive  coloration  in  animals,  Methuen  and  Co., 
Ltd. 

Fig.     4-2.     Simpson,  G.  G.,  1951,  Horses,  Oxford  University  Press. 

Fig.     8-1.     The  quail  were  very  kindly  made  available  by  Dr.  Dwain  Warner, 
Curator  of  Birds,  University  of  Minnesota  Museum  of  Natural  History. 

Fig.  12-1.     Baldwin,  E.,   1949,  An  introduction  to  comparative  biochemistry. 
Cambridge  University  Press.    (Redrawn) 

Fig.   13-2.     Lemche,  H.,  1957.  "A  new  living  deep-sea  mollusc  of  the  Cambro- 

Devonian  class  Monoplacophora,"  Nature  179(1)  :415. 
Fig.  13-4.     Ralph  Buchsbaum. 

Fig.   14-2.     Fuller,  H.  B.,  and  O.  Tippo,  1949,  College  botany,  Holt,  Rinehart 
and  Winston,  Inc. 

Fig.   17-1.     Snyder,  L.  H.,  and  P.  R.  David,  1957,  The  principles  of  heredity, 
5th  ed.,  D.  C.  Heath  and  Company. 

Fig.   17-2.     Srb,  A.,  and  R.  D.  Owen,   1952,  General  genetics,  W.  H.  Free- 
man and  Company. 

Fig.   18-3.     Wilson,  C  L.,  and  W.  E.  Loomis,   1957,  Botany,  rev.  ed.,  Holt, 
Rinehart  and  Winston,  Inc. 

ix 


X  •  CREDITS 

Fig.  21-1.     Edmund  Bert  Gerard,  Cinematographer,  Great  Neck,  N.  Y. 

Fig.  23-2.  Clausen,  J.,  and  W.  M.  Hiesey,  1958,  Experimental  studies  on  the 
nature  of  species,  IV,  Carnegie  Institution  of  Washington. 

Fig.  23-3.  Miintzing,  A.,  1930,  "Uber  Chromosomen-vermehrung  in  Gale- 
op  sis — Kreuzungen  und  ihre  phylogenetische  Bedeutung,"  Hereditas 
14:155. 

Fig.  25-1.  Snyder,  L.  H.,  and  P.  R.  David,  1957,  The  principles  of  heredity, 
5th  ed.,  D.  C.  Heath  and  Company.  (Pictures  from  The  Cattleman) 

Fig.  28-1.  Clausen,  J.,  D.  D.  Keck,  and  W.  M.  Hiesey,  1947,  "Heredity  of 
geographically  and  ecologically  isolated  races,"  Am.  Naturalist  81:114- 
123. 

Fig.  28-2.  Moore,  J.  A.,  1949,  "Patterns  of  evolution  in  the  genus  Rana." 
In  Genetics,  paleontology,  and  evolution,  Jepsen,  G.  L.,  E.  Mayr,  and 
G.  G.  Simpson,  eds.,  Princeton  University  Press. 

Fig.  29-2.     Anderson,  E.,   1949,  Introgressive  hybridization,  John  Wiley  and 

Sons. 
Fig.  29-3.    Manton,    I.,    1934,    "The    problem    of    Biscutella    laevigata,"    L. 

Zeitschr.  f.  ind.  Abst.  n.  Vererbungsl.  67,  Springer- Verlag,  Heidelberg. 

Fig.  31-4.     Lack,  D.,  1947,  Darwin's  finches,  Cambridge  University  Press. 

Fig.  32-1.  Begg,  C.  M.  M.,  1959,  Introduction  to  genetics,  The  Macmillan  Com- 
pany. 

Fig.  32-2.     Stern,  C,  1954,  "Two  or  three  bristles,"  Am.  Sci.  42:284. 

Fig.  32-3.  Snyder,  L.  H.,  and  P.  R.  David,  1957,  The  principles  of  heredity, 
5th  ed.,  D.  C  Heath  and  Company.  (Photograph  by  Dr.  L.  V.  Domm) 

Fig.  33-1.  a,  d,  and  e,  Zoological  Society  of  London,  b,  Walker,  E.  P.,  1954, 
The  monkey  book,  The  Macmillan  Company,  c,  Chicago  Zoological 
Park,  Brookfield,  111. 

Fig.  33-2.  a  and  b,  Walker,  E.  P.,  1954,  The  monkey  book,  The  Macmillan 
Company,  c,  National  Zoological  Park,  Smithsonian  Institution,  Wash- 
ington, D.  C. 

Figs.  33-6  and  33-8.  Washburn,  S.  L.,  I960,  "Tools  and  human  evolution," 
Sci.  American  203(3)  September  I960. 

Fig.  33-7.     a-e,  Peabody  Museum,  Harvard  University. 

Fig.  34-1.  Begg,  C.  M.  M.,  1959,  Introduction  to  genetics,  The  Macmillan  Com- 
pany. 

Fig.  34-2.  Sax,  K.,  1950.  "The  effects  of  x-rays  on  chromosome  structure," 
/.  Cell.  Comp.  Physiol.  35,  Suppl.  1. 

Fig.  35-1.     Sax,  K.,  1955,  Standing  room  only,  Beacon  Press. 

Fig.  35-2.  World  population  and  resources,  1955,  P.  E.  P.  16,  Queen  Anne's 
Gate,  London. 

Fig.  35-3.  Van  Loon,  H.  W.,  1932,  Van  Loon's  geography,  Simon  and 
Schuster,  Inc. 


Contents 


chapter    i    Adaptation    

TYPES  OF  ADAPTATION  4 

THE    ENVIRONMENT  5 

ADAPTATION  IN  THE  FROG  6 

PROTECTIVE   COLORATION  10 

ADAPTATION  IN   MAN  12 

chapter    2    Evolutionary  Thought  before 
Darwin 

GREEK   THOUGHT  14 

THE  DECLINE  OF  SCIENCE  16 

THE    RENAISSANCE  17 

THE   NATURAL   PHILOSOPHERS  17 

BIOLOGICAL  RESEARCH  AND  WRITINGS 


PART  I 

Introduction 
3 


14 


18 


XI 


xil  •  CONTENTS 

chapter    3    Darwin  and  after  Darwin 25 


PART  // 
The  Evidence  for  Evolution 
chapter    4    The  Fossil  Record 39 

RECONSTRUCTING  THE  PAST  40 

EXTINCTION  AND  EVOLUTION  42 

VERTEBRATE   EVOLUTION  44 

EVOLUTION   OF  THE   HORSE  46 


CHAPTER    5    The  Origin  of  the  Earth  and  of 
the  Universe 


51 


AGE   OF  THE   UNIVERSE  51 

NATURE   OF  THE   UNIVERSE 


53 


CHAPTER    6    The  Origin  of  Life 57 

SPONTANEOUS   GENERATION  57 

THE  COMPOSITION  OF  LIVING  THINGS  60 

FORMATION    OF    ORGANIC    COMPOUNDS  6l 

SOURCES  OF  ENERGY  AND  FOOD  64 

chapter    7    Geographical  Distribution 68 

BIOGEOGRAPHICAL   REALMS  69 

PRIMITIVE  AND  MODERN  MAMMALS  IN 

THE    NEOTROPICAL  72 

NEARCTIC  AND   PALEARCTIC  73 

RELICT  ALPINE  POPULATIONS  73 

PRIMITIVE  SOUTHERN  FAUNA  74 

CONTINENTAL  AND  OCEANIC  ISLANDS  75 


79 


chapter    8    Systematics    .  . 

CLASSIFICATION 

VARIATION  80 

THE   BINOMIAL   SYSTEM  81 

SOME  TAXONOMIC  PROBLEMS 


79 


84 


chapter    9    Comparative  Embryology 87 

VON  baer's  dicta         88 

MODIFICATIONS   OF   DEVELOPMENT  90 


CONTENTS  •  Xlll 

chapter  10    Comparative  Anatomy 95 

HOMOLOGY  AND  ANALOGY  95 

HOMOLOGIES   IN   VERTEBRATES  97 

GENETIC  HOMOLOGY  100 

VESTIGIAL   ORGANS  101 

CHAPTER  ii    Comparative  Biochemistry   103 

PLANT   PIGMENTS  105 

PHOTORECEPTORS  106 

IMMUNOLOGY  110 

CHAPTER  12    Biochemical  Adaptation    113 

AQUATIC   LIFE  114 

TERRESTRIAL    LIFE  117 

NITROGEN   EXCRETION  120 

chapter  13    Evolution  in  Animals 123 

PROTOZOA  124 

PORIFERA  125 

MESOZOA  128 

COELENTERATA  128 

CTENOPHORA  129 

PLATYHELMINTHES  129 

ORIGIN  OF  THE  METAZOA  130 

NEMERTEA  132 

ACANTHOCEPHALA  132 

NEMATODA  133 

NEMATOMORPHA,   KINORHYNCHA, 

AND    PRIAPULIDA  133 

GASTROTRICHA  AND  ROTIFERA 
ENTOPROCTA   AND    ECTOPROCTA 
BRACHIOPODA  AND  PHORONIDA 
MOLLUSCA  135 

ANNELIDA  136 

SIPUNCULIDA  AND  ECHIURIDA 
ONYCOPHORA,  A  LIVING  LINK 
ARTHROPODA  139 

CHAETOGNATHA  AND  POGONOPHORA  139 

ECHINODERMATA  140 

HEMICHORDATA  l4l 

CHORDATA  142 


134 
134 

135 


137 
137 


XIV  •  CONTENTS 


chapter  14    Evolution  in  Plants 144 

CYANOPHYTA  146 

RHODOPHYTA  146 

PYRROPHYTA  AND   CHRYSOPHYTA  146 

PHAEOPHYTA  148 

EUGLENOPHYTA  AND  CHLOROPHYTA  148 

SCHIZOMYCOPHYTA  148 

MYXOMYCOPHYTA    AND    EUMYCOPHYTA  149 

OVERLAPPING   SYSTEMS   OF 
CLASSIFICATION  149 

BRYOPHYTA  150 

TRACHEOPHYTA  151 

ORIGIN  OF  VASCULAR  PLANTS  151 

chapter  15    Genetic  Evidence 155 

HYBRIDIZATION  155 

DOMESTICATED   SPECIES  158 

GENE  AND  CHROMOSOME  HOMOLOGY  159 

1    *""   THE  HEREDITARY  MATERIAL  160 

PART  III 

The  Mechanism  of  Evolution 

chapter  16    Mendel's  Laws 166 

^    SEGREGATION  168 

INDEPENDENT  ASSORTMENT  173 

chapter  17    Variation  Due  to  Recombination  .  .    177 

MULTIPLE  ALLELES  177 

BACKGROUND   EFFECTS  180 

RECOMBINATION    AND    INTERACTION  180 

chapter  is    The  Physical  Basis  of  Evolution  ...    185 

'    MITOSIS  185 

LIFE  CYCLE  IN  ANIMALS  188 

LIFE    CYCLE    IN    PLANTS  189 

MEIOSIS  190 

SEX  DETERMINATION  191 

SEX  LINKAGE  191 


CONTENTS  •  XV 


chapter  19    Linkage 195 

'     LINKAGE  AND  CROSSING  OVER  195 

"     LINEAR  ORDER  OF  THE  GENES  196 

chapter  20    Chromosomal  Variation 199 

"DUPLICATION  AND  DEFICIENCY  199 

INVERSION  201 

*  TRANSLOCATION  202 
»      POSITION    EFFECT  AND 

PSEUDOALLELISM  203 

HETEROPLOIDY  204 

'    POLYPLOIDY  204 

chapter  21    Mutation 207 

■     TYPES  OF  MUTATIONS  207 

INDUCED  MUTATION  209 

MUTATION   RATES  210 

CONTROLLED  GENETIC  CHANGES  212 

THE  MUTATION  THEORY  OF  DE  VRIES  213 

chapter  22    Quantitative  Inheritance 216 

•  GENETICS    OF    QUANTITATIVE    TRAITS  218 
,   HETEROSIS            220 

chapter  23    Variation  in  Natural  Populations  .  .   225 

GENETIC  ANALYSIS  OF  NATURAL 
POPULATIONS  226 

V  CHROMOSOMAL    VARIATION  229 

chapter  24    Genetics  of  Populations 234 

THE   HARDY-WEINBERG   EQUILIBRIUM  235 

'     MUTATION  237 

chapter  25    Natural  Selection 239 

ARTIFICIAL   SELECTION  24l 

SELECTION  FOR  RESISTANCE  242 

THE  BALDWIN  EFFECT  244 

THE  THEORY  OF  SELECTION  245 

SELECTION   AND   MUTATION  247 


XVI  •  CONTENTS 


chapter  26    Polymorphism    249 

TRANSIENT   POLYMORPHISM  250 

^THE  ORIGIN  OF  DOMINANCE  252 

BALANCED   POLYMORPHISM  254 

HETEROSIS  AND   POLYMORPHISM  256 

chapter  27    Genetic  Drift 262 

EFFECTIVE  SIZE  OF  POPULATIONS  264 

GENETIC  DRIFT  AND  EVOLUTION  266 

chapter  28    The  Origin  of  Subspecies 268 

POPULATION  STRUCTURE  268 

ISOLATION  AND  SUBSPECIATION  271 

GENETIC  DIFFERENCES   BETWEEN 
SUBSPECIES  272 

chapter  29    Hybridization  and  Evolution 277 

THE  EFFECTS  OF  MIGRATION  278 

INTROGRESSIVE   HYBRIDIZATION  280 

POLYPLOIDY  AND  EVOLUTION  281 

chapter  30    Isolating  Mechanisms   286 

TYPES  OF  ISOLATING  MECHANISMS  286 

THE  ORIGIN  OF  ISOLATING 
MECHANISMS  289 

chapter  31    The  Origin  of  Species 291 

THE  SPECIES  AS  A  BIOLOGICAL  UNIT  292 

MODES    OF    EVOLUTION  293 

THE  ORIGIN  OF  HIGHER  TAXONOMIC 
GROUPS  296 

CHAPTER  32    Evolution  of  Genetic  Systems 301 

GENETIC  RECOMBINATION  301 

^ASEXUAL  VERSUS  SEXUAL 
REPRODUCTION  303 

^    HAPLOIDY  VERSUS  DIPLOIDY  304 

^THE  SEPARATION  OF  THE  SEXES  306 

V  SEX  DETERMINATION  306 

SEXUAL  DIFFERENTIATION  310 

THE  CONTROL  OF  RECOMBINATION  313 

SEXUAL   SELECTION  315 


CONTENTS  •  XV11 


PART  IV 
Evolution  and  Man 


chapter  33    Human  Evolution 

THE   PROSIMIANS  323 

THE   HIGHER   PRIMATES  325 

FOSSIL   PRIMATES  330 

THE  FOSSIL  RECORD  OF  MAN 
THE  ORIGIN  OF  MODERN  MAN 
MAN,  A  POLYTYPIC  SPECIES 
THE  RACES  OF  MAN  343 

CULTURAL   EVOLUTION  345 


323 


332 

338 
341 


chapter  34    Radiation,  Genetics,  and  Man  ....   350 

THE  FREQUENCY  OF  HARMFUL  GENES  350 

GENETIC  EFFECTS  OF  RADIATION  353 

SOMATIC  EFFECTS  OF  RADIATION  355 

RADIATION  EFFECTS  IN  MAN  356 

chapter  35    Man  as  a  Dominant  Species 360 

ELEMENTARY   DEMOGRAPHY  363 

THE  CAUSES  OF  OVERPOPULATION  366 

THE  REGULATION  OF  MAN'S 

INCREASING  NUMBERS  368 


chapter  36    Man's  Future 373 

man's  future  as  a  species        373 
man's  future  numbers        374 
man's  genetic  future        375 
eugenics        376 

Appendix    379 

A:     from  Charles  darwin's  Voyage  of  the 
Beagle         381 

B:     from  thomas  malthus'  Essay  on  the  Principle 
of  Population         389 

Glossary    399 

Index 409 


PART 


Introduction 


CHAPTER 


1 


Adaptation 


In  this  world  are  many  strange  and  wondrous  sights, 
but  the  one  that  most  easily  arouses  a  sense  of  the  ludicrous 
nature  of  things  is  the  slightly  balding,  slightly  paunchy,  slightly 
middle-aged  father  bouncing  on  his  knee  a  baldish,  pot-bellied 
infant,  a  replica  of  himself  not  only  in  general  but  in  many  par- 
ticulars. This  is  the  joke  he  has  played  on  encroaching  old  age, 
and  around  the  process  by  which  it  has  come  to  pass  has  always 
hung  an  aura  of  mystery,  myth,  taboo,  superstition,  and  mirth. 
Despite  the  intense  interest  man  has  always  shown  in  his  own 
self -duplication,  only  in  the  last  century  has  any  real  progress 
been  made  toward  an  understanding  of  the  process.  The  sight  of 
doting  parents  and  their  offspring  raises  still  broader  questions, 
however.  How  far  back  into  the  mists  of  antiquity  does  this  living 
chain  extend?  What  was  its  beginning?  And  how  far  into  the 
future  will  it  persist  ?  Here,  too,  knowledge  has  accumulated  at  an 
accelerating  pace  during  the  past  century.  In  many  ways,  our 
knowledge  and  understanding  of  heredity  and  evolution  have 
developed  hand  in  hand,  for  the  physical  basis  of  heredity  is  also 
the  physical  basis  of  evolution.  But  man  is  only  one  species.  He 
lives  on  a  ball  of  matter  spinning  in  space  and  populated  by  bil- 
lions of  individuals  belonging  to  millions  of  different  species,  as 
diverse  in  nature  as  bacteria  and  orchids,  honey  bees  and  humans. 
This  situation  seems  very  improbable,  for  a  living  organism  ap- 
pears to  contradict,  even  to  defy,  the  ordinary  laws  of  chemistry, 
physics,  and  thermodynamics.  The  question  is,  What  is  the  origin, 
the  history,  and  the  future  of  this  great  variety  of  individualized 


3 


4  •  INTRODUCTION 

protoplasm?  We  cannot  hope  at  present  to  know  all  of  the  answers,  but  our 
knowledge  has  increased  to  the  point  where  we  now  know  something  of  what 
has  happened  in  the  past  and  of  the  mechanisms  responsible  for  the  changes  that 
have  occurred. 

The  physical  evidence  for  the  study  of  this  question  consists  of  the 
species  of  animals  and  plants  now  living  and  of  the  fossils,  which  are  the  rem- 
nants or  traces  of  organisms  that  have  lived  in  the  past.  For  the  moment,  let  us 
consider  the  living  species.  One  feature  common  to  the  great  variety  of  living 
things  is  that  they  are  adapted  for  life  in  the  environment  in  which  they  are 
found.  Obviously,  if  they  were  not  adapted  to  their  environment,  they  would  not 
be  found  there;  they  simply  could  not  survive.  However,  each  species  is  adapted 
to  a  somewhat  different  set  of  environmental  conditions  from  every  other  species. 
Not  only  are  fish  found  in  water,  monkeys  in  trees,  and  antelope  on  the  prairie, 
but  each  different  species  of  fish  tends  to  have  its  own  habitat,  as  any  good  fisher- 
man (or  ichthyologist,  for  that  matter)  will  testify.  Adaptation  is  so  universal 
and  so  self-evident  that  we  tend  to  overlook  or  to  ignore  it,  but  it  is  a  basic  bio- 
logical fact.  Each  living  organism  has  a  particular  set  of  adaptations  peculiarly 
suited  to  its  mode  of  life.  In  fact,  the  adaptations  are  so  precise  in  so  many  cases 
that  they  appear  exactly  suited  to  the  needs  of  the  organism  in  its  environment. 
A  fish,  for  example,  in  order  to  move  about  in  the  water  in  which  it  lives,  obvi- 
ously needs  appendages  such  as  the  fins.  To  speak  of  the  "needs"  of  the  organ- 
ism, however,  is  to  run  the  risk  of  being  teleological.  Such  usage,  which  often  is 
a  reflection  of  a  way  of  thinking,  has  considerably  hampered  the  study  of  adapta- 
tion. Just  because  an  organism  is  constructed  in  a  certain  way  or  behaves  in  a 
certain  way  is  no  indication  that  it  necessarily  has  any  recognition  of  its  needs  or 
that  any  conscious  purpose  or  plan  governs  it.  On  the  other  hand,  lack  of  recog- 
nition of  its  needs  by  the  organism  does  not  indicate  a  lack  of  functional  signifi- 
cance in  its  structure  or  behavior.  A  fin  is  for  swimming,  and  a  wing  for  flying, 
entirely  aside  from  the  question  of  needs  or  cognition. 

Types  of  Adaptation 

Two  general  types  of  adaptation  may  be  distinguished.  One  type  might 
be  called  individual  adaptation,  by  which  an  organism,  through  suitable  modi- 
fications in  its  physiology,  adjusts  to  environmental  stresses.  Fair-skinned  people, 
for  example,  when  exposed  to  sunlight,  typically  become  "tanned."  Even  though 
this  change  is  an  individual  response  to  a  particular  stimulus,  it  is  ultimately 
under  the  control  of  that  person's  hereditary  make-up  or  genotype,  for  not  all 
people  have  the  ability  to  form  melanin  in  response  to  exposure  to  sunlight. 
Albinos  and  people  with  very  light  complexions  may  continue  to  sunburn  despite 
continued  exposure  to  the  sun;  the  ability  to  tan  is  simply  beyond  the  capacity  of 
their  genotypes.  The  discomfort  of  such  people  could  be  considered  sufficient  evi- 


ADAPTATION  •   5 

dence  of  the  adaptive  value  of  the  ability  to  tan,  but  it  would  be  desirable  to 
know  more  about  the  process.  On  the  other  hand,  some  human  populations  are 
much  more  heavily  pigmented  than  others,  the  pigment  developing  even  though 
the  individuals  may  not  be  exposed  to  the  sun.  In  the  dark-skinned  races,  pig- 
ment is  formed  under  the  control  of  the  genotype  also,  but  no  external  stimulus 
is  needed.  In  these  races,  population  adaptation  may  be  said  to  exist,  for  the 
whole  population  routinely  has  darkly  pigmented  skin.  There  seems  little  reason 
to  doubt  that  the  skin  pigment  of  the  dark-skinned  races  has  adaptive  value  just 
as  it  does  in  the  case  of  individual  adaptation,  but  the  exact  nature  of  this  adap- 
tive value  at  present  remains  a  matter  of  speculation.  The  two  types  of  adapta- 
tion, individual  and  population,  are  rather  different  although  both  are  under 
hereditary  control.  One  of  the  more  intriguing  questions  in  evolutionary  research 
is  how  individual  adaptation  may  be  transformed  into  population  adaptation.  It 
may  seem  to  verge  on  the  question  of  the  inheritance  of  acquired  characteristics 
but  is  nonetheless  quite  distinct  from  it. 

Although  each  species  is  unique  in  its  adaptations  to  its  own  particular 
physical  and  biological  environment,  nevertheless  all  species  face  essentially  the 
same  basic  problems.  The  variety  of  different  kinds  of  adaptations  represent  dif- 
ferent solutions  to  these  problems.  For  example,  oxygen  is  required  in  the 
metabolism  of  fish  and  mammals  (and  most  other  species) ;  the  fish  extract 
oxygen  from  the  water  through  their  gills,  but  the  mammals  use  quite  different 
structures — the  lungs — to  obtain  oxygen  from  air.  The  basic  problems  confront- 
ing every  species,  if  it  is  to  continue  to  exist,  are  very  simple:  it  must  survive, 
and  it  must  reproduce.  In  order  to  survive,  an  organism  must  obtain  an  adequate 
supply  of  food;  it  must  have  some  measure  of  protection  from  other  organisms, 
whether  predators,  competitors,  or  parasites;  and  it  must  make  suitable  adjust- 
ments to  the  existing  physical  conditions.  Survival  alone  is  not  enough,  however. 
If,  at  a  given  time,  all  the  members  of  one  species  survived  through  maturity  to 
old  age  without  reproducing,  that  species  would  become  extinct  with  that 
generation. 

No  adaptation  is  perfect.  With  the  variety  of  functions  required  of  the 
organism,  the  adaptations  achieved  must  be,  perforce,  a  compromise  among  all 
these  functions.  The  organism  is  a  complex  bundle  of  adjustments  to  its  neigh- 
bors of  all  degree  and  to  its  physical  environment. 

The  Environment 

The  nature  of  the  environment  is  worthy  of  comment,  for  it  will  em- 
phasize the  variety  of  adaptations  required  for  survival  and  reproduction.  The 
physical  environment  consists  of  some  sort  of  substrate;  this  may  be  fresh  or  salt 
water,  or  land,  or  air,  or,  for  the  parasites,  another  organism.  Fresh  water  alone 
represents  a  variety  of  substrates  requiring  somewhat  different  adaptations  for 


6  •  INTRODUCTION 

survival — in  lakes,  rivers,  streams,  ponds,  swamps,  and  so  on — whereas  each 
different  species  represents  a  different  substrate  for  the  parasites.  Another  limit- 
ing physical  factor  is  temperature.  Different  species  may  have  somewhat  different 
ranges  of  temperature  tolerance,  but  the  actual  range  at  which  any  life  as  we 
know  it  is  possible  is  really  rather  narrow.  Strangely  enough,  this  range  happens 
to  coincide  with  existing  temperatures  on  the  earth.  Other  forces  such  as  pressure 
and  gravity  are  a  constant  part  of  the  environment.  Furthermore,  sound  waves, 
light  waves,  and  chemical  particles  are  constantly  impinging  upon  the  organism. 
The  biotic  environment  of  an  organism  consists,  first,  of  other  members 
of  the  same  species,  which  interact  with  each  other  in  various  ways.  In  relation 
to  reproduction  there  may  be  courtship  and  care  of  the  young.  There  may  also  be 
various  group  activities — colony  formation  or  migration,  for  example — requiring 
some  degree  of  cooperation.  Competition  between  members  of  the  same  species 
may  develop  in  the  quest  for  food  or  in  the  establishment  of  nesting  territories. 
Many  adaptations  appear  to  be  related  to  these  functions.  Furthermore,  the  rela- 
tions between  different  species  may  be  as  diverse  as  predation,  parasitism,  compe- 
tition, and  cooperation. 

Adaptation  in  the  Frog 

Thus  far,  our  discussion  has  been  rather  general,  and  it  may  be  helpful 
to  consider  briefly  the  problems  of  adaptation  as  they  have  been  solved  by  one 
species.  The  leopard  frog,  Ran  a  pipiens,  has  been  widely  used  in  zoological 
laboratories  in  the  United  States.  Because  it  is  so  familiar,  the  frog  is  especially 
suitable  for  reappraisal  here  in  terms  of  its  adaptations  rather  than  of  its  organ 
systems.  In  so  doing,  we  may  seem  to  belabor  the  obvious. 

To  survive,  the  frog  is  confronted  with  the  problem  of  finding  and 
securing  an  adequate  supply  of  food.  To  move  about  in  this  search,  the  frog  has 
legs,  which  are  adapted  for  swimming  in  water  and  for  jumping  on  land.  The 
webbed  feet  are  obvious  adaptations  for  swimming.  However,  since  the  legs 
function  for  locomotion  in  or  on  two  media,  they  represent  an  adaptive  com- 
promise, and  it  is  quite  clear  that  the  frog  is  not  very  efficient  at  moving  about 
in  either.  His  search  for  food  is  guided  by  the  major  sense  organs  of  sight,  hear- 
ing, smell,  and  taste,  which  serve  as  receptors  of  more  or  less  distant  stimuli.  It 
is  a  rather  remarkable  fact,  though  you  may  not  at  first  so  consider  it,  that  all  of 
these  major  sense  organs  are  localized  in  the  head,  which  is  at  the  front  end  of 
his  bilaterally  symmetrical  body.  (Bilateral  symmetry — that  is,  an  arrangement  of 
the  body  into  anterior  and  posterior  ends,  and  dorsal  and  ventral  surfaces — is  an 
adaptation  to  an  active  life.  Sessile  species  are  generally  radially  symmetrical; 
that  is,  their  body  parts  are  arranged  about  a  central  axis.)  It  would  seem  quite 
a  coincidence  that  these  sense  organs  are  so  strategically  placed  at  the  anterior 
end,  which  is  constantly  probing  into  new  parts  of  the  environment.  Imagine 


ADAPTATION  •   7 

how  much  less  useful  these  structures  would  be  if  arranged  on  the  frog's 
posterior. 

Once  the  food  has  been  located,  the  mouth  assumes  the  problem  of 
securing  it.  The  tongue,  unlike  man's,  is  attached  at  the  front  of  the  mouth 
cavity  and  is  flicked  out  with  speed  and  precision  to  pick  off  unwary  insects  that 
come  within  reach.  The  vomerine  teeth,  in  the  roof  of  the  mouth,  crush  the 
insects  before  they  pass  into  the  digestive  tract.  In  the  digestive  system,  the  food 
is  broken  down  into  molecules  that  can  be  absorbed  through  the  walls  of  the 
intestine  and  transported  by  the  circulatory  system  to  the  immediate  vicinity  of 
the  individual  living  cells.  The  respiratory  system  is  also  tied  in  with  the  circu- 
latory system  so  that  the  oxygen  essential  for  the  utilization  of  the  food  mole- 
cules during  the  metabolic  activity  of  the  cells  is  made  available  to  them.  The 
waste  products  of  cellular  metabolism  are  in  turn  removed  by  the  circulatory 
system,  carbon  dioxide  (C02)  being  eliminated  primarily  from  the  lungs  and 
nitrogenous  wastes  by  the  kidneys.  The  frog's  digestive  system,  respiratory  sys- 
tem, circulatory  system,  and  excretory  system  are  fundamental  adaptations  for 
supplying  the  necessary  metabolic  raw  materials  to  the  living  cells  and  removing 
the  waste  products  after  the  cells  have  extracted  energy  and  essential  compounds 
from  them.  Without  adaotations  of  this  sort,  multicellular  life  would  not  be  at 
all  possible. 

Furthermore,  the  organism  acts  as  an  integrated  whole,  not  merely  as  a 
collection  of  cells,  tissues,  and  organs.  This  integration  is  due  to  chemical  co- 
ordinating systems,  mainly  hormonal,  and  to  the  nervous  system.  As  a  result,  the 
individual  cells  become  interacting  and  interdependent  parts  of  a  well-integrated 
unit.  These  chemical  and  nervous  mechanisms  operate  in  such  a  way  that  even 
under  stress  a  balanced  internal  environment  is  maintained.  Maintenance  of  an 
internal  dynamic  equilibrium  is  called  homeostasis. 

There  are  several  ways  in  which  the  frog  secures  some  degree  of  protec- 
tion from  other  organisms.  The  sense  organs  and  the  locomotor  system  obviously 
serve  a  dual  purpose,  in  securing  food  and  escaping  predators.  The  dorsal  place- 
ment of  the  eyes  and  nostrils  is  adaptive  in  that  the  frog  can  remain  almost 
completely  submerged  in  water,  and  yet  it  can  breathe  and  see  above  the  surface. 
Placement  of  the  eyes  in  the  skull  is  an  adaptive  feature,  as  can  be  easily  ob- 
served by  comparing  the  angles  of  vision  in  a  carnivore  like  the  cat  and  an 
herbivore  such  as  the  rabbit. 

The  coloration  of  the  leopard  frog  has  considerable  protective  value. 
The  basic  color  is  a  cryptic  green  or  greenish  brown,  matching  the  tall  grass  or 
weeded  bank  that  is  the  frequent  habitat  of  this  species.  By  its  ability  to  regulate 
the  degree  of  dispersion  of  the  pigment  granules  in  its  chromatophores,  the  frog 
is  capable  of  considerable  change  in  shade  to  match  its  background.  Moreover, 
the  outline  of  the  body  is  broken  up  by  the  numerous  spots  on  the  skin.  This  so- 
called  disruptive  pattern  destroys  the  visual  impression  that  would  otherwise  be 


8  •  INTRODUCTION 

gained  of  the  frog's  size  and  shape,  and  it  is  especially  effective  when  observed 
(or  not  observed)  in  the  pattern  of  light  and  shadow  created  in  a  grassy  meadow 
on  a  sunny  morning.  Even  to  details,  the  disruptive  effect  is  much  in  evidence; 
the  eye  is  masked  to  some  extent  by  a  dark  line  that  seems  to  run  through  it,  and 
the  matching  up  of  the  spots  on  the  upper  and  lower  parts  of  the  hind  legs 
creates  a  series  of  dark  bands  running  at  right  angles  to  the  length  of  the  long 
bones,  disrupting  the  outline  of  these  otherwise  quite  prominent  appendages.  It 
should  be  noted  that  all  of  this  coloration  is  found  only  on  the  dorsal  surfaces  of 
the  body;  the  ventral  surfaces  are  creamy  white.  This  pattern  of  dark  above  and 
light  below  is  known  as  countershading,  and  its  adaptive  significance  lies  in  the 
fact  that  the  frog  when  seen  from  below  in  the  water  will  be  very  light,  match- 
ing the  sky.  (For  a  most  interesting  and  authoritative  account  on  the  functional 
significance  of  animal  coloration,  see  Cott's  Adaptive  Coloration  in  Animals.) 
In  addition  to  its  concealing  function,  the  skin  serves  as  a  more  or  less  effective 
barrier  to  infection  by  a  variety  of  parasites  and  as  a  respiratory  organ. 

The  frog  is  a  rather  stupid  animal  with  quite  stereotyped  behavior.  It 
escapes  the  notice  of  its  predators  by  remaining  motionless;  if  alarmed  suffi- 
ciently, it  gives  a  series  of  explosive  leaps  and  then  once  again  freezes.  If  it 
jumps  into  the  water,  it  burrows  into  the  mud  or  debris  for  concealment.  These 
behavior  patterns,  though  simple,  are  clearly  adaptive  for  the  protection  of  the 
frog  from  predators.  However,  leopard  frogs  appear  to  have  a  rather  complex 
pattern  of  migratory  behavior.  In  the  spring  they  migrate  to  the  breeding  ponds, 
and  then,  after  breeding,  apparently  move  on  to  summer  feeding  territories.  In 
the  fall,  as  colder  weather  ensues,  large-scale  migrations  to  over-wintering  sites  in 
lakes  and  streams  take  place.  These  migratory  patterns  are  clearly  adaptive. 

In  winter,  the  air  temperature  drops  below  the  range  at  which  the  frogs 
can  remain  active,  and  to  survive,  they  burrow  into  the  debris  at  the  bottom  of 
ponds  and  streams.  Other  controlling  physical  factors  in  the  life  of  the  frog 
include  moisture.  Though  leopard  frogs  seem  less  closely  tied  to  damp  areas  than 
most  other  amphibian  species,  it  is  clear  that  this  species  too  may  be  subject  to 
dehydration  rather  quickly.  Certainly  they  are  much  less  in  evidence  in  open 
meadows  on  sunny,  dry,  and  windy  days  than  on  cloudy  and  humid  days. 

Perhaps  the  most  remarkable  adaptations  of  all  are  those  related  to 
reproduction.  Reproduction  in  the  leopard  frog  occurs  in  the  spring  in  rather 
shallow  pools.  The  calling  males  congregate  in  large  numbers  at  the  breeding 
site.  The  females  are  attracted  to  the  site,  deposit  their  eggs  while  clasped  in 
amplexus  by  the  males,  and  depart.  The  physical  and  biological  factors  that 
initiate  and  control  this  elaborate  series  of  events  are  in  most  instances  matters 
of  conjecture;  for  example,  we  do  not  know  what  determines  the  selection  of 
the  breeding  site,  which  must  not  dry  up  before  the  tadpoles  metamorphose.  The 
obvious  differences  between  males  and  females  are  not  great,  the  nuptial  pads 
and  the  song  of  the  males  during  the  breeding  season  being  the  most  noticeable. 


ADAPTATION  •  9 


Fig.  1-1.     Life  cycle  of  the  leopard  frog,  Rana  pipiens. 


At  every  stage  in  the  life  cycle  (Fig.  1-1),  adaptations  appear.  The  egg  mass  of 
pipiens  from  the  warm  southern  parts  of  the  United  States  is  rather  flattened, 
whereas  that  of  females  from  the  northern  states  is  globular;  there  is  an  obvious 
relation  to  the  lower  oxygen  concentration  in  warm  water  as  compared  to  cold. 
The  eggs  are  countershaded.  The  larva  that  emerges  is  an  aquatic  animal,  swim- 
ming with  fins  and  respiring  with  gills.  Unlike  the  adult,  it  is  an  herbivore,  with 
its  digestive  tract  correspondingly  adapted  for  handling  this  different  type  of 
food.  The  remarkable  series  of  changes  known  as  metamorphosis  then  occurs, 
with  the  adult  frog,  a  terrestrial  tetrapod,  the  result. 


10  •  INTRODUCTION 

These,  then,  are  some  of  the  adaptations  in  the  frog;  for  the  most  part 
they  are  not  particularly  striking  or  unusual.  The  frog  was  chosen  as  an  example 
to  illustrate  the  fact  that  even  the  most  familiar  species  is  quite  precisely  adapted 
to  its  ecological  niche.  As  species  go,  the  leopard  frog  must  be  regarded  as  sort 
of  a  fringe  dweller,  firmly  established  neither  in  water  nor  on  land.  Yet  in  this 
marginar^rrvkonment,  which  is  its  normal  habitat,  the  frog  has  been  quite  suc- 
cessful by  the  only*  criterion  we  have  for  measuring  biological  success — that  is, 
survival  as  a  species.  Mere  survival  may  not  seem  at  first  glance  to  be  a  very  lofty 
criterion  by  which  to  judge  success,  but  at  least  it  is  objective.  Certainly  this 
evolutionary  line  has  outlasted  some  more  impressive  and  dominant  species  that 
have  lived  in  the  past,  such  as  the  mammoth,  the  saber-toothed  tiger,  and  all  of 
the  dinosaurs. 

Protective  Coloration 

The  discussion  of  adaptation  sometimes  tends  to  dwell  on  the  more 
spectacular  types  of  adaptive  changes,  some  of  which — among  them,  protective 
coloration — are  extremely  fascinating.  The  adaptive  value  of  animal  colors  has 
sometimes  been  doubted.  For  example,  when  the  Nile  catfish  was  found  to  show 
reversed  countershading — that  is,  the  dorsal  surface  light  and  the  ventral  surface 
dark — the  whole  theory  of  countershading  was  brought  under  suspicion.  How- 
ever, the  concept  was  doubly  strengthened  when  it  was  discovered  that  this  fish 
characteristically  swims  upside  down. 

Not  only  are  colors  frequently  adapted  for  concealment  of  the  organ- 
ism, but  the  animal  may  enhance  the  protective  value  of  its  coloration  by  its 
behavior.  Certain  moths  are  cryptically  colored  to  match  the  bark  on  which  they 
ordinarily  rest,  and  in  addition  they  hold  their  wings  flat  against  the  bark,  which 
eliminates  the  shadow,  and  position  their  bodies  in  such  a  manner  that  their 
pattern  best  matches  the  pattern  of  the  bark  (Fig.  1-2). 

The  coloration  of  some  animals  is  adapted  not  so  much  for  concealment 
by  blending  in  with  the  background  of  its  habitat  as  it  is  for  disguise,  by  which 
they  resemble  some  other  object  in  their  environment.  A  number  of  species — for 
example,  butterflies  and  other  insects,  fish,  and  frogs — resemble  leaves;  still 
others  resemble  twigs  or  lichens.  A  most  peculiar  group  are  the  geometrid  moths 
that  resemble  bird  droppings,  especially  startling  when  they  fly  away.  There  is 
much  in  common  between  the  desert  lizard,  which  lures  unwary,  insects  to  their 
deaths  because  the  corner  of  its  mouth  when  opened  resembles  a  small  red  desert 
flower,  and  the  anglerfish,  which  has  a  dorsal  spine  modified  into  a  lure  that 
dangles  before  its  gaping  mouth. 

In  some  species  the  so-called  aposematic  colors  serve  as  advertisements 
rather  than  as  disguise  or  concealment.  The  skunk,  with  his  striking  black  and 
white  colors,  is  not  easily  missed  nor  is  he  easily  mistaken  for  any  other  species. 
The  white  flag  of  the  Virginia  white-tailed  deer  appears  to  serve  as  a  warning 


ADAPTATION  •   11 


Fig.  1-2.     Willow  beauty  moth  (Boarmia  gemmaria)  resting  on  bark.   Con- 
cealment is  achieved  by  the  similarity  between  the  wing  pattern  and  the 
bark,  and  is  further  enhanced  by  the  horizontal  positioning  of  the  body  and 
the  elimination  of  shadows  from  the  wings.    (Courtesy  of  Cott.) 


signal.  In  birds,  the  same  colors  used  by  the  male  as  a  part  of  the  courtship  dis- 
play may  also  be  used  in  a  threat  display  toward  other  males  invading  his 
territory. 

The  insects  with  stings,  such  as  bees,  wasps,  and  hornets,  are  usually 
strikingly  colored  black  and  yellow  and  tend  to  some  extent  to  resemble  each 
other.  This  type  of  mimicry,  in  which  a  number  of  dangerous  or  unpalatable 
species  resemble  one  another,  is  known  as  Mullerian  mimicry.  This  is  distinct 


12  •  INTRODUCTION 

from  Batesian  mimicry,  in  which  the  harmless  species  resemble  the  harmful  or 
nauseous  types.  A  classical  example  of  mimicry  is  the  resemblance  of  the  Viceroy 
butterfly  {LlmenUis  archippus)  to  the  Monarch  (Danaus  plexippus).  The 
Viceroy  is  colored  orange  and  black  like  the  Monarch  and  is  quite  different  from 
the  other  members  of  its  own  genus,  which  are  black  with  white  spots.  Originally 
thought  to  be  a  case  of  Batesian  mimicry,  this  example  may  not  fit  either  classical 
pattern,  for  recent  evidence  has  shown  that  the  Viceroy,  though  more  palatable 
to  birds  than  the  Monarch,  is  eaten  somewhat  less  often  than  other  butterflies. 
The  whole  subject  of  mimicry  is  of  extreme  interest,  and  much  work  remains  to 
be  done  to  clarify  many  of  the  questions  in  this  field. 

Examples  of  remarkable  adaptations  could  be  cited  almost  endlessly, 
but  only  one  more  will  be  mentioned.  A  certain  shrike  in  Ceylon  (Hemipus 
picatus)  builds  its  nest  on  the  bare  limbs  of  trees.  The  nest  is  so  constructed  that 
it  resembles  a  knot,  and  is  cunningly  camouflaged  with  bits  of  bark  and  lichen 
to  heighten  the  effect.  The  young  birds  are  cryptically  colored  so  that  they  blend 
with  the  nest.  Most  remarkable  of  all,  however,  is  the  fact  that  the  birds  sit 
facing  each  other  with  their  eyes  partially  closed  and  their  beaks  pointing  up- 
ward and  almost  touching.  The  total  effect  of  the  cooperative  efforts  of  parents 
and  young  is  that  of  a  knot  on  a  dead  branch  with  just  a  small  stub  of  a  broken 
branch  protruding  from  the  knot.  To  visualize  how  these  complex  behavior  pat- 
terns became  incorporated  into  the  hereditary  make-up  of  this  species,  as  they 
clearly  must  be,  is  to  stretch  the  imagination. 

Adaptation  in  Man 

Although  we  tend  to  think  of  man  as  having  mastered  his  environment, 
actually  he  is  adapted  to  rather  specific  environmental  conditions,  and  his  mastery 
is  due  to  his  skill  in  modifying  the  environment  to  approximate  his  needs  rather 
than  in  broadening  his  environmental  tolerances.  Man  is  a  terrestrial  animal,  and 
was  undoubtedly  confined  to  the  tropics  and  subtropics  until  his  relatively  recent 
discovery  of  the  use  of  fire  and  clothing.  His  erect  bipedal  locomotion  is  adapted 
to  life  in  relatively  open  country  rather  than  to  heavily  forested  or  mountainous 
regions.  His  lungs  enable  him  to  extract  oxygen  from  the  air,  and  he  requires 
an  adequate  daily  supply  of  fresh  drinking  water.  Though  an  omnivore,  he  is 
ultimately  dependent  on  green  plants  for  all  of  his  food.  This  analysis  could  be 
extended,  but  it  should  suffice  to  demonstrate  that  man,  too,  makes  well-defined 
demands  on  his  environment. 

SUGGESTED  READING 

Carpenter,  G.  D.  H.,  and  E.  B.  Ford,  1933.  Mimicry.  London:  Methuen. 
Caspari,  E.,  1951.  "On  the  biological  basis  of  adaptedness,"  Am.  Scientist,  39:441- 
451. 


ADAPTATION  •   13 

Cott,  H.  B.,  1940.  Adaptive  coloration  in  animals.  New  York:  Oxford  University 
Press. 

Emerson,  A.  E.,  I960.  "The  evolution  of  adaptation  in  population  systems,"  Evolu- 
tion after  Darwin,  Vol.  I,  The  evolution  of  life,  S.  Tax,  ed.  Chicago:  Uni- 
versity of  Chicago  Press. 

Huxley,  J.  S.,  1943.  Evolution.  The  modern  synthesis.  New  York:  Harper. 

Muller,  H.  J.,  1950.  "Evidence  of  the  precision  of  genetic  adaptation,"  Harvey  Lec- 
tures, 43:165-229. 

Portmann,  A.,  1959.  Animal  camouflage,  A.  J.  Pomerans,  tr.  Ann  Arbor:  University 
of  Michigan  Press. 

Simpson,  G.  G.,  1953.  The  major  features  of  evolution.  New  York:  Columbia  Uni- 
versity Press. 

Stephenson,  E.  M.,  and  C.  Stewart,  1955.  Animal  camouflage,  2d  ed.  London:  Black. 

Waddington,  C.  H.,  I960.  "Evolutionary  adaptation,"  Evolution  after  Darwin, 
Vol.  I,  The  evolution  of  life,  S.  Tax,  ed.  Chicago:  University  of  Chicago 
Press. 


CHAPTER 


Evolutionary  Thought 
before  Darwin 


Although  thought  on  the  origin  of  species  has  apparently 
preoccupied  men  of  almost  every  culture,  much  of  the  speculation 
has  been  of  such  a  nature  that  it  must  be  regarded  as  based  largely 
on  myth,  superstition,  or  vague  philosophical  ideas  rather  than  on 
careful  observation  and  the  accumulation  of  facts.  Furthermore, 
the  modern  reader  may  read  into  the  statements  of  earlier  writers 
things  they  did  not  intend  to  say.  In  this  short  review  we  obvi- 
ously cannot  hope  to  trace  the  complete  history  of  the  develop- 
ment of  the  evolution  concept.  Instead  a  sampling  of  the  ideas 
advanced  at  different  periods  will  be  presented  in  an  effort  to 
convey  some  of  the  flavor  of  the  thinking  of  different  ages. 

Greek  Thought 

Among  the  Greeks,  Anaximander,  who  lived  in  the 
sixth  century  B.C.  (611-547  B.C.)  merits  attention,  for  he  at- 
tempted to  explain  the  origin  of  the  universe  on  a  rational  basis 
rather  than  by  myths  or  legends.  He  visualized  all  things  as  hav- 
ing come  from  a  primordial  fluid  or  slime  to  which  they  ulti- 
mately return.  Living  things,  both  plant  and  animal,  were  formed 
as  this  mud  dried.  This  concept  appears  to  be  one  of  the  earliest 
known  theories  of  spontaneous  generation.  Man  himself  was  first 
shaped  like  a  fish  and  lived  in  the  water.  Later,  when  he  became 
capable  of  terrestrial  life,  he  burst  forth  from  his  fishlike  capsule 

14 


EVOLUTIONARY   THOUGHT   BEFORE    DARWIN  •   15 

like  a  butterfly  from  its  chrysalis  and  assumed  human  form  and  a  life  on  land. 
This  theory  was  crude,  yet  the  implication  of  evolution  is  clear. 

Xenophanes  (576P-480  B.C.),  believed  to  have  been  a  pupil  of  Anaxi- 
mander,  is  the  first  person  known  to  have  recognized  that  fossils  were  the  rem- 
nants of  once-living  organisms  and  that  marine  fossils  on  land  indicated  that  the 
sea  formerly  covered  the  earth. 

Empedocles  in  the  fifth  century  B.C.  (495-435  B.C.)  stated  that  the  four 
elements  were  air,  earth,  fire,  and  water,  and  that  these  elements  were  acted  upon 
by  two  forces,  love  and  hate,  which  caused  their  union  or  separation.  He  also 
suggested  that  plants  had  arisen  first,  and  that  animals  were  later  formed  from 
them.  The  germ  of  the  idea  of  natural  selection  was  contained  in  his  belief  that 
the  parts  of  animals  were  formed  separately  and  then  united  at  random  by  the 
triumph  of  love  over  hate.  Most  would  then  be  monsters  and  unviable,  but  a 
few  could  survive.  He  and  many  others,  both  before  him  and  for  centuries  after- 
ward, believed  in  the  possibility  of  spontaneous  generation  of  life  from  nonliving 
materials,  and  thus  settled,  in  rather  simple  fashion,  the  question  of  the  origin  of 
life. 

Aristotle  (384-322  B.C.),  whose  ideas  dominated  biological  thought  for 
well  over  a  thousand  years,  was  the  greatest  of  the  Greek  men  of  science.  He 
was  a  vitalist,  believing  that  living  things  were  animated  by  a  vital  force  or 
guiding  intelligence  quite  different  from  anything  to  be  found  in  nonliving 
matter.  In  this  idea  he  was  preceded  by  Anaxagoras  (500-428  B.C.),  but  to 
Aristotle  this  internal  force  became  a  perfecting  principle,  operating  constantly 
to  improve  or  perfect  the  living  world.  Growing  out  of  this  concept  was  his 
ladder  of  nature  ("Scala  naturae")  or  chain  of  being  in  which  he  arranged  living 
things  on  a  scale  of  perfection.  The  succession  ranged  from  inanimate  matter 
through  the  lower  plants  to  the  higher  animals  on  a  single  scale  with  man,  at  the 
top,  being  the  most  nearly  perfect.  Aristotle  apparently  never  interpreted  the 
chain  as  possibly  suggesting  that  each  group  had  evolved  from  the  one  below  it. 
He  believed  in  spontaneous  generation  not  only  for  smaller  animals  but  for 
larger  ones  such  as  frogs  and  snakes.  He  thought  that  the  inheritance  of  mutila- 
tions was  rather  common,  but  rejected  the  idea  of  the  inherited  effects  of  use  and 
disuse.  Adaptation  to  him  was  the  result  neither  of  the  survival  of  accidental 
fitness,  as  it  was  for  Empedocles,  nor  of  functional  modifications  but  rather  of 
the  action  of  the  perfecting  principle.  Thus  Aristotle  did  not  add  in  any  direct 
way  to  the  development  of  modern  evolutionary  thought  despite  his  many  con- 
tributions to  biology.  Since  he  remained  the  most  authoritative  source  of  biolog- 
ical information  for  so  long  a  period,  it  could  be  argued  that  some  of  his  theories 
actually  hampered  the  development  of  the  theory  of  evolution.  The  difficulties, 
however,  lay  less  with  Aristotle  than  with  the  nature  of  the  times  that  followed 
him. 

For  centuries  after  Aristotle  little  progress  was  made  toward  a  better 


16  •  INTRODUCTION 

understanding  of  evolution,  for  the  spirit  of  inquiry  that  characterized  the  Greeks 
gradually  withered  away  and  died.  Epicurus  (341-270  B.C.)  is  worth  mention- 
ing, not  because  he  added  significantly  to  evolutionary  thought  but  because  he 
attempted  to  explain  the  world  and  the  universe  as  natural  phenomena  governed 
by  natural  causes.  As  a  materialist  or  mechanist,  he  tried  to  combat  the  super- 
stitious beliefs  in  supernatural  forces  ruling  the  universe.  In  this  effort  he  op- 
posed the  Aristotelian  argument  of  teleology,  or  the  grand  design  or  purposeful- 
ness  of  events,  which  was  widely  accepted  at  the  time.  As  a  part  of  his  philos- 
ophy he  adopted  the  atomic  theory  of  Democritus  (460P-362?  B.C.). 

The  Decline  of  Science 

The  Roman  poet  Lucretius  (99-55  B.C.)  was  a  follower  of  Epicurus, 
and  in  his  famous  work,  On  the  Nature  of  Things  (De  Rerum  Natura),  summed 
up  most  of  the  Greek  non-Aristotelian  thought.  Lucretius  is  significant,  not  for 
any  particular  advance  in  evolutionary  thought,  but  because  he  marked  the  end 
of  a  period  of  thought,  and  through  his  work  preserved  the  atomic  theory  during 
the  Dark  and  Middle  Ages  and  gave  a  forceful  restatement  of  the  mechanistic 
position.  In  his  rejection  of  Aristotle's  teleology,  he  also  rejected  much  of  the 
rest  of  Aristotle's  work,  and  thus  did  not  achieve  a  complete  synthesis  of  the 
best  of  Greek  thought. 

The  Roman  Pliny  (a.d.  23-79)  compiled  a  tremendous  store  of  infor- 
mation and  misinformation  in  his  Natural  History,  which  served  as  man's 
primary  source  of  knowledge  about  natural  history  for  nearly  1500  years.  He 
was  not  primarily  an  investigator,  however,  and  his  uncritical  recitation  of  the 
work  of  others  added  nothing  new.  Galen  (a.d.  130-200),  the  last  important 
biologist  of  antiquity  and  the  personal  physician  of  Marcus  Aurelius,  made  in- 
vestigations in  anatomy  and  physiology  that  were  accepted  as  authoritative  for 
centuries,  but  he,  too,  made  no  direct  contribution  to  evolutionary  theory.  Thus, 
at  the  close  of  the  classical  period  some  few  ideas  that  had  a  bearing  on  evolution 
had  been  expressed,  but  the  concept  was  far  from  its  modern  form. 

Although  the  decline  of  ancient  science  has  at  times  been  attributed  to 
the  rise  of  Christianity,  this  seems  hardly  to  have  been  the  case.  The  decline  set 
in  long  before  the  birth  of  Christ  and  even  at  the  time  of  Galen's  death,  in 
a.d.  200,  the  Christians  were  only  a  small  group  without  influence.  Preoccupa- 
tion with  spiritual  matters  did  little  to  advance  science,  and  active  conflicts  did 
develop  later,  but  no  one  church  can  claim  any  monopoly  on  this  sort  of  opposi- 
tion. For  centuries  the  churches  were  the  primary  centers  of  learning.  Such  lead- 
ers among  the  early  Christians  as  St.  Augustine  (354-430)  and  much  later 
St.  Thomas  Aquinas  (1225-1275),  who  has  remained  an  authority  of  the 
Church,  rejected  a  literal  interpretation  of  the  story  of  special  creation  in  Genesis 
and  suggested  instead  an  allegorical  naturalistic  interpretation  patterned  after 
Aristotle.  However,  throughout  the  Dark  Ages  no  progress  was  made  in  the 


EVOLUTIONARY   THOUGHT    BEFORE    DARWIN  •   17 

development  of  the  theory  of  evolution.  The  rise  of  Scholasticism  in  the  thir- 
teenth century  led  to  the  study  of  the  writings  of  the  ancients  on  nature  but  to 
little  study  of  nature  itself.  Much  of  this  material  was  obtained  from  translations 
of  works  in  Arabic,  many  of  which  had  in  turn  been  derived  from  the  Greek.  In 
the  reaction  by  the  Church  in  1209  against  Arabian  science  and  philosophy,  the 
study  of  Aristotle  was  also  banned,  but  this  interdiction  was  later  relaxed.  This 
period  marked  the  beginning  of  the  trend  toward  a  literal  interpretation  of  the 
seven  days  of  creation,  a  trend  that  predominated  for  centuries.  The  Spanish 
Jesuit  Suarez  (1548-1617)  was  among  those  who  argued  strongly  in  favor  of  a 
literal  interpretation  of  Genesis  and  refuted  Augustine  and  Thomas  Aquinas. 
The  result  was  that  for  three  centuries,  from  the  sixteenth  to  the  middle  of  the 
nineteenth,  Special  Creation  was  official  Church  doctrine  even  though  it  was  a 
departure  from  the  beliefs  of  some  of  the  earlier  leaders  of  Christianity.  Diver- 
sity of  opinion  was  denounced  as  heresy,  and  free  discussion  of  the  concept  of 
evolution  carried  with  it  the  risk  of  reprimand  or  excommunication  by  the 
Church  even  up  to  the  time  of  Buffon  in  the  late  eighteenth  century.  Whether 
this  attitude  aided  or  hindered  the  development  of  the  theory  of  evolution  is 
hard  to  say,  but  it  did  play  a  significant  part  in  the  history  of  the  concept. 

The  Renaissance 

The  revival  of  the  classical  art  and  learning  of  the  Greeks  and  Romans, 
which  was  known  as  the  Renaissance,  took  place  during  the  fourteenth,  fifteenth, 
and  sixteenth  centuries.  This  development,  in  turn,  led  to  a  rebirth  in  the  spirit 
of  inquiry;  the  Renaissance  was  not,  however,  marked  by  any  notable  progress 
on  the  question  of  the  origin  of  species.  Leonardo  da  Vinci  (1452-1519)  real- 
ized that  the  fossil  marine  shells  that  he  found  in  the  Apennine  mountains  indi- 
cated that  they  must  once  have  been  covered  by  the  sea,  but  he  did  not  develop 
the  idea  in  relation  to  biological  evolution.  Similarly,  Cesalpino  (1519-1603) 
suggested  that  flower  petals  were  modified  leaves,  another  concept  that  could 
have  led  to  the  theory  of  evolution.  Most  of  the  naturalists  of  the  time  were 
Encyclopedists  who  made  every  effort  to  collect  all  the  known  facts  about  living 
things.  The  discovery  by  Harvey  (1578-1657)  of  the  circulation  of  the  blood  in 
a  sense  marks  the  transition  from  the  biology  of  the  ancients  to  modern  experi- 
mental biology. 

The  Natural  Philosophers 

In  the  seventeenth  and  eighteenth  centuries  a  number  of  men  now 
known  as  the  natural  philosophers  tried  to  develop  unified  systems  of  thought 
by  which  they  could  interpret  the  universe.  Since  life  is  a  part  of  the  universe, 
biological  matters  were  included  in  their  schemes  of  things.  Although  their  inter- 
ests were  not  always  primarily  biological,  they  did  make  some  advances  in  evolu- 
tionary thought.  We  will  mention  here  just  some  of  the  biological  insights  of  a 


18  •  INTRODUCTION 

few  of  these  men.  Francis  Bacon  (1561-1626)  called  upon  men  to  seek  knowl- 
edge by  observation,  experiment,  and  inductive  reasoning,  and  to  free  themselves 
from  both  Scholasticism  and  Aristotelean  philosophy.  He  strongly  urged  that 
the  variations  in  nature  should  be  studied  and  their  causes  determined.  Further- 
more, he  pointed  out  that  artificial  selection  among  these  variations  could  be 
used  to  cause  species  to  change  and  that  transitional  forms  exist  in  nature.  Al- 
though his  examples  were  somewhat  farfetched — he  suggested,  for  example,  that 
flying  fishes  were  intermediate  between  fishes  and  birds,  and  bats  between  birds 
and  quadrupeds — the  fact  remains  that  even  at  the  opening  of  the  seventeenth 
century  the  question  of  the  fixity  of  species  was  being  raised. 

Bacon  proposed  methods  by  which  the  nature  of  the  universe  could  be 
determined,  but  Descartes  (1596-1650)  was  the  pioneer  among  the  systematic 
philosophers  who  speculated  on  the  nature  of  the  system  itself.  Guarded  in  his 
expression,  he  postulated  that  the  universe  could  be  explained  on  physical  prin- 
ciples. This  mechanistic  approach  had  a  great  impact  on  biology,  especially  since 
it  came  just  after  Harvey's  success  in  explaining  the  circulation  of  the  blood  in 
physical  terms.  Descartes  was  circumspect  in  presenting  his  ideas  out  of  fear  of 
offending  the  Church,  and  his  writings  on  physiology,  which  became  the  founda- 
tions of  modern  physiology,  were  withheld  from  publication  until  after  his  death. 
Since  he  spoke  in  terms  of  the  evolution  of  the  universe,  and  life  was  a  part  of 
this  system,  the  evolution  of  life  was  more  or  less  indirectly  included. 

Leibnitz  (1646-1716)  had  a  better  scientific  background  than  his 
predecessors,  for  he  understood  the  nature  and  origin  of  fossils,  had  extensive 
knowledge  of  plant  and  animal  classification  and  of  comparative  anatomy,  and 
was  familiar  with  the  wonders  revealed  by  the  recently  discovered  microscope. 
His  doctrine  of  continuity  applied  to  life  was  still  another  revival  of  the  Aris- 
totelean chain  of  being,  but  it  did  not  necessarily  lead  him  to  the  concept  of  evo- 
lution. He  did,  however,  speculate  on  the  relationship  between  the  fossil  am- 
monites and  the  living  nautilus  and  even  suggested  that  major  changes  of  habitat 
might  cause  changes  in  animal  species.  He  stated  that  his  doctrine  of  continuity 
led  to  the  idea  that  intermediate  species  should  exist,  but  he  shied  away  from 
the  thought  of  species  intermediate  between  man  and  the  apes,  saying  that  if 
they  existed,  it  must  be  in  another  world.  Kant  (1724-1804),  who  has  often 
been  cited  as  a  predecessor  of  Darwin,  was  undoubtedly  familiar  with  the  sug- 
gestion that  species  change  but  he  apparently  never  embraced  the  idea  of  evolu- 
tion completely. 

Biological  Research  and  Writings 

Just  as  the  natural  philosophers  influenced  the  thought  and  direction  of 
research  of  the  biologists  of  their  day,  they,  in  turn,  were  influenced  by  the  ad- 
vances being  made.  One  such  advance  was  the  development  of  a  system  of  classi- 


EVOLUTIONARY   THOUGHT   BEFORE    DARWIN  •   19 

fication  for  plants  and  animals.  The  foremost  predecessor  of  Linnaeus  (1707- 
1778),  who  is  universally  regarded  as  the  father  of  the  modern  binomial  system 
of  nomenclature,  was  John  Ray  (1627-1705),  an  English  naturalist.  Ray  wrote  a 
number  of  systematic  works,  primarily  on  plants  but  also  on  animals,  that  repre- 
sented major  advances  toward  the  "natural  system"  of  classification,  which  takes 
into  account  all  known  similarities  and  differences.  It  was  Ray  who  first  clearly 
defined  the  species  concept  as  being  related  to  community  of  descent  and  inter- 
fertility  rather  than  to  fixity  of  type,  but  he  did  not  extend  this  idea  in  the 
direction  of  evolution.  Linnaeus  himself  in  the  tenth  edition  (1758)  of  his 
Systema  Naturae  established  the  foundation  on  which  taxonomy  has  since  been 
built.  His  scheme  was  a  branching  one,  rather  than  a  chain  or  ladder  form,  and 
living  things  were  named  according  to  genus  and  species — man,  for  example, 
being  Homo  sapiens.  Althought  he  developed  a  branching  system,  Linnaeus  at 
first  believed  in  the  fixity  of  species;  as  his  experience  broadened,  however,  he 
came  in  later  editions  to  accept  the  possibility  of  evolution,  at  least  within  the 
genus,  due  either  to  hybridization  or  the  effects  of  environment. 

The  work  of  de  Maupertius  (1698-1759)  has  recently  been  rescued 
from  an  undeserved  obscurity.  Eminent  in  his  own  day,  he  aroused  the  wrath  of 
Voltaire,  whose  bitter  mockery  has  undoubtedly  colored  the  opinions  of  posterity. 
His  arguments  against  the  preformation  doctrine  in  embryology  preceded  those 
of  Wolff  by  fifteen  years.  Moreover,  he  developed  a  particulate  theory  of  heredity 
based  on  experiments  in  animal  breeding  and  investigations  of  human  heredity, 
applying  probability  theory  to  his  findings  a  century  before  Mendel.  In  addition 
to  foreshadowing  nearly  all  aspects  of  Mendelian  genetics,  he  developed  a  theory 
of  evolution  based  on  mutation,  selection,  and  geographic  isolation.  In  this  work 
he  was  so  far  ahead  of  his  time  that  it  is  perhaps  not  surprising  that  his  theories 
were  not  understood  or  appreciated. 

The  evolutionary  writings  of  Buff  on  (1707-1788),  one  of  the  most  in- 
fluential biologists  of  the  eighteenth  century,  have  been  variously  interpreted — 
perhaps  because  they  were  so  widely  scattered  among  his  extensive  works.  There 
can  be  little  doubt  that  Buffon  influenced  the  thinking  of  his  successors  about 
evolution,  but  it  is  not  entirely  clear  whether  he  himself  ever  developed  a  con- 
sistent theory  of  evolution  in  which  he  believed  wholeheartedly.  One  factor  was 
his  concern  not  to  arouse  the  displeasure  of  the  ecclesiastical  authorities.  How- 
ever, he  did  state  parts  of  the  theory  of  organic  evolution  in  considerable  detail, 
and  his  writings  thus  served  as  the  starting  point  for  much  of  the  subsequent 
work.  Among  his  contributions  were  several  of  significance.  He  anticipated 
Malthus,  concerning  the  relation  between  population  and  food  supply.  He  called 
attention  to  the  fundamental  similarities  between  animals  of  quite  different 
species,  thus  giving  impetus  to  the  study  of  comparative  anatomy,  now  a  corner- 
stone in  the  evidence  for  evolution.  His  recognition  of  variation  within  species 
and  of  the  possibility  of  gradual  change  within  species  giving  rise  to  new 


20  •  INTRODUCTION 

varieties  seems  very  modern.  The  similarities  between  apes  and  men,  the  horse 
and  the  ass,  made  him  raise  the  question  of  their  relations  to  one  another.  His 
suggestion  that  the  apes  and  the  ass  were  degenerate  types  led  to  the  idea  of  a 
common  ancestry.  He  understood  the  significance  of  fossils  and  believed  that  the 
time  scale  needed  to  be  greatly  extended  beyond  the  commonly  accepted  scale  of 
his  day.  These  and  many  other  portions  of  his  works  indicate  the  modern  lines 
along  which  his  thinking  was  progressing.  On  the  other  hand,  many  passages 
could  be  cited  to  indicate  that  he  believed  in  the  immutability  of  species,  a  belief 
that  grew  from  his  use  of  hybrid  sterility  as  the  criterion  for  delimiting  the 
species.  Within  the  species,  he  thought  change  was  possible,  but,  not  visualizing 
a  mechanism  by  which  sterility  might  arise  during  evolution,  he  was  more  or  less 
forced  to  argue  against  large-scale  evolution.  Buffon's  writings  contain  contra- 
dictions, but  they  nevertheless  were  most  influential  in  their  impact  on  subse- 
quent generations. 

Going  back  in  time,  we  find  a  number  of  speculative  authors  dealing 
with  evolution,  of  whom  we  shall  mention  just  one.  De  Maillet  (1656-1738)  in 
Telliamed  drew  together  from  the  science  of  his  day  many  threads  to  weave  his 
theories.  His  unorthodox  views  were  attributed  to  an  Indian  philosopher,  "Telli- 
amed" (De  Maillet  spelled  backward).  Perhaps  his  major  contribution  was  his 
clear  statement  on  the  nature  and  origin  of  fossils,  about  which  varied  opinions 
were  still  held.  In  his  view,  the  gradual  drying  up  of  the  seas  over  long  periods 
of  time  was  responsible  for  marine  fossils  in  the  mountains  and  could  also  ex- 
plain the  similarities  between  aquatic  and  terrestrial  forms,  terrestrial  species 
having  been  transformed  from  marine  animals  trapped  in  marshes.  Many  species 
undoubtedly  failed  to  make  the  transition,  he  thought,  but  from  the  successful 
ones  the  land  animals  and  birds  arose.  When  he  cited  specific  cases,  however,  he 
was  not  so  cogent,  for  he  derived  birds  from  flying  fish,  and  men  and  women 
from  mermen  and  mermaids.  Thus,  he  entangled  facts  with  myths  and  legends, 
and  his  real  contributions  in  the  interpretation  of  fossils  and  rock  stratification 
came  under  suspicion. 

The  uniformitarianism  of  James  Hutton  (1726-1797)  postulated  that 
the  ordinary  forces  of  wind,  water,  heat,  cold,  and  so  forth,  that  we  observe 
today  are  the  same  forces  that  worked  to  reshape  and  restructure  the  earth's  sur- 
face in  the  past,  and  hence  no  mysterious  or  supernatural  phenomena  were  in- 
volved in  these  changes.  If  this  were  the  case,  Hutton  reasoned,  the  earth's  age 
must  be  much  greater  than  previously  imagined  and  the  various  catastrophic 
theories  must  be  wrong.  William  Smith  (1769-1839)  was  primarily  responsible 
for  recognizing  that  each  of  the  different  layers  or  strata  of  rock  has  its  own 
characteristic  types  of  fossils  and  that  the  lower  the  strata,  the  less  the  fossils 
resemble  living  forms.  Charles  Lyell  (1797-1875)  in  his  Principles  of  Geology 
established  the  science  of  geology  in  its  modern  form.  This  work,  published  at 
the  time  of  Darwin's  voyage  on  the  Beagle,  was  of  great  importance  to  the  de- 


EVOLUTIONARY   THOUGHT    BEFORE    DARWIN  •  21 

velopment  of  Darwin's  ideas.  One  of  the  major  effects  of  the  development  of 
geology  on  the  theory  of  evolution  was  that  it  showed  the  existence  of  a  vast 
span  of  time  during  which  evolution  could  have  taken  place. 

Erasmus  Darwin  (1731-1802),  the  grandfather  of  Charles,  is  note- 
worthy not  only  for  that  fact  but  also  because  in  Zoonomia  he  gave  the  first  clear 
statement  of  the  theory  of  the  inheritance  of  acquired  characteristics,  according 
to  which  the  effects  wrought  by  the  environment  on  the  organism  are  thought  to 
be  transmissible  to  the  offspring.  This  theory  was  more  completely  developed 
by  Lamarck  (1744-1829),  with  whose  name  it  is  usually  associated  (Fig.  2-1). 


Lamarck's  early  years  were  spent  in  military  service  until  ill  health  forced  him  to 
resign.  An  interest  in  botany,  acquired  while  stationed  in  Monaco,  led  him  to 
study  medicine,  of  which  botany  was  then  an  important  part.  A  book  on  the  flora 
of  France  established  his  reputation,  won  him  the  friendship  of  Buffon  and  other 
biologists,  and  eventually  gained  him  a  post  as  botanist  at  the  Jardin  du  Roi. 
The  reforms  touched  off  by  the  French  Revolution  included  the  ouster  of  men 
who  had  previously  been  leaders  in  biology,  and  when  two  new  chairs  in  zoology 
were  created,  the  two  most  suitable  candidates  were  Lamarck,  a  botanist  nearing 
fifty,  and  St.  Hilaire,  a  mineralogist.  They  apparently  decided  to  split  the  animal 
kingdom  between  them,  Lamarck  taking  the  invertebrates  and  St.  Hilaire  the 


22  •  INTRODUCTION 

vertebrates.  The  most  remarkable  aspect  of  this  story  is  that  both  men  went  on  to 
distinguished  careers  in  their  new  fields. 

In  Philosophie  Zoologique  (1809)  Lamarck  wrote  more  extensively 
about  the  evidence  for  evolution  than  had  anyone  prior  to  that  time.  His  sug- 
gested mechanism  for  evolution  was  the  inheritance  of  acquired  characteristics. 
He  believed  that  the  activity  of  an  animal  enhanced  the  development  of  the  more 
frequently  used  structures,  producing  modifications  that  were  inherited;  lack  of 
use  led  to  degenerative  changes,  which  were  also  inherited.  St.  Hilaire,  in  sup- 
porting Lamarck,  stressed  the  direct  effects  of  the  environment  as  causes  of 
hereditary  change,  but  Lamarck  accepted  this  theory  only  in  plants.  An  animal's 
need  for  a  structure  might  also  lead  to  its  development — the  long  neck  of  a 
giraffe,  for  example,  being  the  result  of  constant  stretching  over  many  genera- 
tions. Thus,  use  and  disuse,  need,  and  the  direct  effects  of  the  environment  have 
come  to  be  considered  as  basic  concepts  in  the  theory  of  the  inheritance  of  ac- 
quired characteristics. 

Unfortunately,  despite  its  many  appealing  features,  no  critical  evidence 
has  ever  been  produced  in  favor  of  Lamarckianism.  Nevertheless,  this  theory  has 
been  made  the  official  theory  of  heredity  in  the  Soviet  Union  under  the  name  of 
Michurinism.  The  rise  to  power  of  Lysenko,  which  began  in  the  early  1930s  and 
became  complete  in  1948  with  the  abolition  of  teaching  and  research  in  Men- 
delian  genetics,  is  a  most  unusual  story.  The  attack  was  basically  political,  and 
the  geneticists  as  well  as  their  science  were  made  to  suffer.  Despite  its  political 
success,  Lamarck's  theory  of  the  inheritance  of  acquired  characteristics  still  re- 
mains to  be  demonstrated  experimentally,  for  Lysenko' s  experiments  lack  ade- 
quate controls,  do  not  involve  strains  of  known  ancestry,  and  are  not  treated 
statistically  at  all. 

Lamarck's  ideas  on  evolution  were  subjected  to  forceful  criticism  by 
Cuvier  (1769-1832),  who  was  virtually  a  scientific  dictator  in  France  with  un- 
paralleled political  and  scientific  influence.  Cuvier  is  generally  considered  to  be 
the  father  of  two  sciences,  paleontology  and  comparative  anatomy.  However, 
even  though  these  two  fields  now  furnish  some  of  the  most  impressive  evidence 
available  on  the  course  of  evolution,  Cuvier's  work  led  him  to  believe  in  the 
fixity  of  species  and  to  deny  that  evolution  gave  a  satisfactory  interpretation  of 
his  findings.  He  recognized  that  different  rock  strata  contained  different  types  of 
fossils,  but  attributed  the  gaps  in  the  record  to  a  series  of  catastrophes,  following 
which  immigration  of  different  species  from  other  areas  repopulated  the  deva- 
stated regions.  He  believed  the  last  such  catastrophe  to  have  been  the  flood 
recorded  in  Genesis.  His  followers  carried  his  ideas  one  step  further  and  postu- 
lated that  successive  creations  were  responsible  for  the  new  kinds  of  species 
found  after  each  catastrophe.  Although  his  active  opposition  to  Lamarck  and 
St.  Hilaire  certainly  hampered  the  development  and  acceptance  of  the  theory  of 
evolution,  nevertheless  in  one  respect  Cuvier  was  of  great  significance  to  subse- 


EVOLUTIONARY   THOUGHT   BEFORE    DARWIN  •  23 

quent  work.  St.  Hilaire  supported  the  concept  of  the  unity  of  type  among  all 
animal  species — the  old  idea  of  the  scale  of  being  or  ladder  of  nature  that  can 
be  traced  all  the  way  back  to  Aristotle.  In  particular,  he  compared  the  cephalopod 
mollusks,  such  as  the  squid,  with  the  vertebrates.  In  the  controversy  that  broke 
into  the  open  between  St.  Hilaire  and  Cuvier  in  1830,  Cuvier  conclusively  dem- 
onstrated that  no  such  unity  existed  and  thus  cleared  the  ground  for  the  branch- 
ing system  of  divergent  evolution.  Whereas  St.  Hilaire  (and  Lamarck)  were 
right  in  principle  about  evolution  and  wrong  in  detail,  Cuvier  was  wrong  in 
principle  but  right  in  detail  about  the  data  drawn  from  comparative  anatomy. 
Since  his  views  prevailed  on  both  subjects,  the  evolution  theory  undeniably 
suffered. 

Thus  the  idea  of  evolution — that  species  change — was  clearly  not  en- 
tirely original  with  Charles  Darwin.  Nor,  as  Darwin  recorded  in  an  introductory 
historical  sketch  to  the  Origin  of  Species,  was  he  the  first  to  propose  the  theory 
of  natural  selection  as  the  mechanism  of  evolution.  Several  of  his  predecessors 
deserve  mention.  An  expatriate  royalist  American  physician,  William  Wells 
(1757-1817),  appears  to  have  been  the  first  to  enunciate  the  principle  of  natural 
selection  in  a  reasonably  modern  form,  in  a  paper  entitled  "An  account  of  a 
white  female,  part  of  whose  skin  resembles  that  of  a  Negro"  read  in  1813  but 
generally  ignored  at  the  time.  Another  of  Darwin's  predecessors  whom  he  also 
apparently  overlooked  was  Patrick  Matthew.  In  this  case,  Darwin  could  probably 
be  excused,  for  Matthew's  views  on  natural  selection  were  published  in  the  ap- 
pendix of  a  work  entitled  Naval  Timber  and  Arboriculture.  Yet  Matthew,  in  his 
quest  for  recognition,  called  attention  to  his  priority  over  Darwin  in  the  title 
pages  of  his  subsequent  works.  Recently,  still  another  candidate  for  the  honor  of 
discovering  natural  selection  has  been  unearthed  in  the  person  of  Edward  Blyth 
(1810-1873).  It  has  been  suggested  that  Darwin  was  less  than  completely  candid 
in  disclosing  the  extent  of  his  debt  to  his  predecessors,  although  to  what  extent 
this  criticism  is  valid  may  be  very  difficult  to  determine.  Even  though  it  may  be 
established  that  Darwin  had  read  the  papers  of  such  men  as  Blyth  and  Matthew, 
it  would  be  difficult  if  not  impossible  to  learn  whether  he  consciously  drew  on 
them  at  the  time  he  achieved  his  great  synthesis.  Certainly  his  conduct  toward 
Alfred  Russell  Wallace  was  always  both  proper  and  generous. 

The  book  The  Vestiges  of  the  Natural  History  of  Creation  was  anony- 
mously published  by  Robert  Chambers  (1802-1871)  in  1844  and  went  through 
ten  editions  in  nine  years.  Since  Chambers  was  an  amateur  scientist,  his  book 
was  filled  with  errors,  and  scientists  generally  attacked  it  bitterly,  an  attack  in 
which  they  were  joined  by  the  clergy.  Their  vehemence  seemed  to  stimulate 
interest  in  the  book  rather  than  to  kill  it,  however.  The  book  showed  that 
Chambers  was  familiar  with  the  works  of  geologists  such  as  Hutton  and  Smith 
and  of  such  biologists  as  Buffon,  Erasmus  Darwin,  Lamarck,  St.  Hilaire,  and 
Cuvier.  From  them  he  drew  his  arguments  in  favor  of  cosmic  and  biological 


24  •  INTRODUCTION 

evolution  as  opposed  to  special  creation.  The  book  was  not  significant  for  origi- 
nality but  rather  for  the  controversy  and  interest  it  aroused  in  the  subject  of  evo- 
lution. Much  of  the  ire  that  might  have  broken  over  Darwin's  head  had  already 
been  spent  on  Chambers.  That  the  idea  of  evolution  did  not  lack  influential  sup- 
port even  in  the  1850s  just  prior  to  publication  of  the  Origin  of  Species  is  indi- 
cated by  the  1852  essay  of  Herbert  Spencer  (1820-1903)  called  "The  Develop- 
ment Hypothesis."  In  it  for  the  first  time  the  word  "evolution"  was  used  in  the 
general  sense  in  which  it  is  used  today.  Thus  it  should  be  clear  that  the  theories 
of  Darwin  and  Wallace  that  struck  with  such  impact  in  1859  had  a  long  period 
of  development  prior  to  the  synthesis  set  forth  in  the  Origin  of  Species. 

SUGGESTED  READING 

Barlow,  N.,  ed.,  1958.  The  autobiography  of  Charles  Darwin  1809-1882.  London: 
Collins. 

Carter,  G.  S.,  1957.  A  hundred  years  of  evolution.  London;  Sidgwick  and  Jackson. 

Darwin,  C,  1839.  The  voyage  of  the  Beagle.  New  York:  Bantam  Books  (1958). 

,  1872.  On  the  origin  of  species.  New  York:  Mentor  Books  (1958). 

,  and  A.  R.  Wallace,  1958.  Evolution  by  natural  selection.  New  York:  Cam- 
bridge University  Press. 

Eiseley,  L.,  1958.  Darwin's  century.  Garden  City,  New  York:  Doubleday. 

,  1959.  Charles  Darwin,  Edward  Blyth,  and  the  theory  of  natural  selection. 

Proc.  Amer.  Philos.  Soc.  /03.'94-158. 

Glass,  B.,  O.  Temkin,  and  W.  Straus,  Jr.,  eds.,  1959.  Forerunners  of  Darwin.  1745- 
1859.  Baltimore:  Johns  Hopkins  University  Press. 

Grant,  V.,  1956.  "The  development  of  a  theory  of  heredity,"  Am.  Scientist,  44:158- 
179. 

Greene,  J.  C,  I960.  The  death  of  Adam.  Ames:  Iowa  State  College  Press. 

Huxley,  J.  S.,  1949.  Soviet  genetics  and  world  science.  London:  Chatto  and  Windus. 

Irvine,  W.,  1955.  Apes,  angels,  and  Victorians.  New  York:  McGraw-Hill. 

Lovejoy,  H.  O.,  1953.  The  great  chain  of  being.  Cambridge,  Massachusetts:  Harvard 
University  Press. 

Moore,  R.,  1953.  Man,  time,  and  fossils.  New  York:  Knopf. 

Nordenskiold,  E.,  1928.  The  history  of  biology,  L.  B.  Eyre,  tr.  New  York:  Knopf. 

Osborn,  H.  F.,  1929.  From  the  Greeks  to  Darwin,  2d  ed.  New  York:  Scribner's. 

Singer,  C,  1959-  A  history  of  biology,  3d  ed.  New  York:  Abelard-Schuman. 


CHAPTER 


Darwin  and 
after  Darwin 


On  February  12,  1809,  two  of  the  greatest  figures  of 
the  nineteenth  century  were  born,  Abraham  Lincoln  and  Charles 
Darwin.  The  circumstances  surrounding  the  events  could  hardly 
have  been  less  similar.  Lincoln's  start  came  in  a  backwoods  log 
cabin,  whereas  Darwin  was  the  son  of  a  successful,  well-to-do 
physician,  Robert  Darwin,  who  had  married  a  girl  of  the  famed 
Wedgewood  pottery  family.  Thus,  his  family  was  doubly  well  off 
financially,  and  Charles  later  further  insured  his  financial  status 
by  marrying  his  first  cousin,  another  Wedgewood.  As  he  put  it  in 
his  autobiography,  "I  have  had  ample  leisure  from  not  having  to 
earn  my  own  bread."  This,  then,  is  one  route  to  making  great 
scientific  discoveries,  but  it  should  be  noted  that  the  names  of 
many  others  as  well  off  financially  as  Darwin  are  now  lost  in 
obscurity. 

Darwin  was  quite  a  normal  boy.  He  liked  to  fish  and 
hunt,  to  collect  almost  anything,  but  not  to  attend  school.  His 
training  at  Dr.  Butler's  school  consisted  of  classics  exclusively, 
and  he  was  considered  by  both  his  teachers  and  his  father  as  a 
little  below  average  in  intelligence.  His  liking  for  mathematics 
and  chemistry,  and  his  interests  in  collecting  insects  and  minerals 
were  not  satisfied  in  school.  It  seemed  logical  that  he  should  fol- 
low in  the  footsteps  of  his  father  and  grandfather  before  him  and 
study  medicine.  For  this  purpose,  he  went  to  Edinburgh,  but  soon 
dropped  this  course  of  study.  A  major  reason  was  his  revulsion 


25 


26  •  INTRODUCTION 

at  some  of  the  more  gory  and  hideous  scenes  a  medical  man  was  expected  to 
endure  in  those  days  before  anesthesia.  During  his  stay  in  Edinburgh  he  became 
acquainted  with  people  who  were  interested  in  geology  and  natural  history,  and 
his  own  interests  were  aroused  to  the  point  where  he  took  courses  at  the  Univer- 
sity in  these  subjects.  Unfortunately,  as  too  often  happens,  formal  instruction 
quickly  killed  this  interest. 

His  father,  apparently  fearing  that  his  son  was  never  going  to  amount 
to  anything  and  seeking  some  sort  of  respectable  career  for  him,  then  suggested 
that  he  go  to  Cambridge  to  study  for  the  clergy.  Charles  was  quite  amenable  to 
this  suggestion,  and  went  to  Cambridge  where,  in  due  course,  he  received  his 
degree,  having  achieved  no  particular  distinction  and  having  made  no  great 
efforts  in  his  studies.  In  fact,  most  of  his  energies  were  devoted  elsewhere,  for 
he  was  an  ardent  hunter  and  horseman,  and  in  the  evenings,  in  a  gentlemanly 
way,  he  sowed  his  wild  oats,  drinking  and  playing  cards.  Small  wonder  that  his 
father  thought  that  the  cloak  of  respectability  of  a  clergyman  might  help  to  keep 
his  son  from  becoming  a  well-to-do  ne'er-do-well. 

At  this  time  his  scientific  inclinations  were  slightly  manifest  in  his 
attendance  at  lectures  in  botany  by  Henslow,  in  his  beetle  collecting  (he  once 
was  confronted  by  three  unusual  specimens  and  freed  a  hand  to  try  for  the  third 
by  tossing  one  into  his  mouth),  and  in  his  friendship  with  distinguished  scien- 
tists such  as  Henslow  and  the  geologist  Sedgwick.  This  last  aspect  of  his  behavior 
was  perhaps  the  most  unusual.  It  is  rather  rare  for  a  young  college  student  to 
seek  friendship  among  the  professors,  and  it  is  perhaps  even  more  rare  to  find 
the  professors  accepting  as  a  friend  one  who  had  so  far  shown  no  particular 
promise.  It  is  to  their  credit  that  the  professors  apparently  saw  something  in  him. 
Out  of  his  friendship  with  the  botanist  Henslow  came  the  event  that  changed 
and  shaped  the  entire  subsequent  course  of  Darwin's  life,  for  Henslow  recom- 
mended him  for  the  position  of  naturalist  without  pay  on  the  Beagle  (Fig.  3-1), 
a  ship  that  was  to  make  a  long  cruise  around  the  world,  charting  many  little- 
known  areas  (Fig.  3-2).  After  some  discussion  with  his  family,  Charles  accepted, 
and  his  career  in  the  clergy  was  never  again  seriously  considered. 

The  voyage  lasted  five  years,  for  the  Beagle  made  many  long  stops,  and 
much  of  the  time  was  spent  in  South  American  waters.  Darwin's  account  of  his 
adventures,  The  Voyage  of  the  Beagle,  is  a  most  fascinating  and  readable  book, 
much  more  so  than  the  closely  argued  Origin  of  Species.  It  is  obvious  that  his 
experiences  on  this  trip  started  the  chain  of  thought  that  ultimately  led  to  his 
theories  of  evolution.  The  course  of  his  work  gave  him  his  first  insight  into  the 
relations  between  species.  He  observed  at  first  hand  how  species  changed  as  one 
traveled  from  north  to  south  in  South  America;  he  observed  the  character  of 
island  faunas;  and  he  saw  the  relations  between  the  fossils  he  discovered  and  the 
existing  species  in  the  same  areas.  He  not  only  observed,  but  he  made  extensive 
and  systematic  collections  of  living  and  fossil  materials.  The  facts  of  species 


DARWIN  AND  AFTER  DARWIN  •  27 


TV  X 


Fig.  3-1.     The  Beagle,  the  vessel  in  which  Charles  Darwin  sailed  around  the  world. 


variation,  of  geographic  distribution,  and  of  the  fossil  record  were  almost  forced 
to  his  attention. 

Unfortunately,  the  weak  stomach  that  had  contributed  to  the  ending  of 
his  medical  studies  still  plagued  him,  and  he  was  seasick  a  good  part  of  this  five- 
year  period.  In  fact,  through  the  rest  of  his  life,  he  was  unable  to  stand  any  sort 
of  excitement,  for  it  almost  inevitably  led  to  digestive  disturbances.  Even  having 
friends  for  dinner  and  a  quiet  talk  afterward  was  enough  to  lead  to  discomfort 
and  sleeplessness.  A  modern  diagnosis  would  probably  suggest  that  his  troubles 
were  psychosomatic,  but  nevertheless  they  were  severe  and  sometimes  incapaci- 
tated him  for  months  at  a  time  in  later  life. 

Upon  his  return  to  England  in  1836,  Darwin  started  to  work  on  his 
collections  and  to  write  up  the  results  of  his  travels.  At  the  same  time  he  began 
to  collect  all  kinds  of  data  bearing  on  the  question  of  the  transmutation  of 
species.  He  carefully  recorded  all  of  the  arguments  both  for  and  against,  being 
especially  careful  to  put  down  quickly  those  against,  for  he  found  that  he  could 
very  conveniently  forget  them.  In  October  1838  he  read  for  the  first  time 
Malthus'    "Essay   on   Population,"    an   excerpt   from  which   is   found   in   Ap- 


28  •  INTRODUCTION 


England' 


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F*>.  3-2.     The  voyage  of  the  Beagle  (1831-1836) 


DARWIN    AND   AFTER    DARWIN  •  29 

pendix  B.  Here  was  a  clue  as  to  the  mechanism  by  which  species  change.  In 
Malthus'  discussion  of  the  reproductive  potential  of  man  being  greater  than  the 
power  of  the  earth  to  produce  subsistence  Darwin  saw  the  essence  of  the  struggle 
for  existence  that  led  to  the  theory  of  natural  selection. 

He  first  wrote  out  his  ideas  on  the  origin  of  species  in  rough  form  in 
1842  and  a  more  complete  draft  was  drawn  up  in  1844,  but  he  continued  to 
assemble  facts  until  1856.  Then,  urged  on  by  Lyell,  he  started  to  write  up  his 


MARINE  IGUANA 


Chatham 


Charlt 


Hood 


Fig.  3-3.     The  Galapagos  Islands  and  route  of  the  Beagle. 


material  in  a  work  that  he  expected  to  fill  four  volumes.  This  undertaking  was 
nowhere  near  completion  when  he  received  a  manuscript  from  Alfred  Russel 
Wallace,  then  in  the  Malay  Archipelago,  who  asked  him  to  read  it  and,  if  he 
thought  well  of  it,  to  send  it  on  to  Lyell  for  his  opinion.  The  paper  contained, 
in  complete  detail,  the  theory  of  natural  selection. 

The  subsequent  events  tend  to  restore  one's  faith  in  human  nature. 
Jealousy  over  priority  among  scientists  is  fairly  common,  yet  the  attitudes  of 
Darwin  and  Wallace  at  this  time  and  for  the  rest  of  their  lives  were  exceedingly 
generous.  Darwin  sent  the  paper  on  with  praise  and  the  recommendation  that  it 
be  published  at  once.  Lyell  and  the  botanist  Hooker,  aware  of  the  long  years 


30  •  INTRODUCTION 

Darwin  had  spent  in  developing  the  theory,  insisted  that  Wallace's  paper  and 
an  extract  of  Darwin's  manuscript  and  one  of  his  letters  written  to  the  American 
botanist,  Asa  Gray  of  Harvard,  should  be  published  simultaneously.  This  was 
done,  with  the  papers  appearing  in  1858.  The  projected  four- volume  work  was 
abandoned  by  Darwin,  who  condensed  his  material  into  a  single  volume,  the 
famed  Origin  of  Species,  which  appeared  the  following  year.  This  work  was  an 
immediate  success  and  had  terrific  impact  not  only  on  the  scientific  world  but  on 
the  world  at  large,  in  contrast  to  the  reception  of  the  original  papers. 


Fig.  3-4.     Charles   Darwin   in    1840,   two  years 

prior  to  his  first  draft  of  the  theory  of  evolution 

by  natural  selection.     (From  a  water  color  by 

George  Richmond.) 

The  circumstances  under  which  Wallace  arrived  at  the  theory  of  natural 
selection  were  rather  similar  to  those  that  initiated  Darwin's  trend  of  thought. 
Wallace  was  a  naturalist  whose  travels  among  the  islands  of  the  East  Indies 
impressed  on  him  the  differences  between  species  as  well  as  their  obvious  rela- 
tionships to  each  other,  and  led  him  to  evolution  and  natural  selection  as  the 
explanation  for  his  observations.  It  seems  as  if  biological  knowledge  had  reached 
the  point  where  an  adequate  training  and  extensive  field  work  led  almost  in- 
evitably to  the  major  synthesis  that  Darwin  and  Wallace  achieved  independently. 

In  his  book  Darwin  actually  presented  evidence  bearing  on  two  distinct 
subjects:  the  theory  of  evolution,  and  a  theory  of  the  mechanism  of  evolution — 


DARWIN   AND   AFTER   DARWIN  •   31 

that  is,  natural  selection.  Darwin  proposed  on  the  one  hand  that  evolution  had 
occurred,  that  existing  species  are  descended  from  similar  but  somewhat  different 
species  that  lived  in  the  past.  The  evidence  he  presented  came  from  his  study  of 
variation  under  domestication  and  in  nature,  from  taxonomy,  from  comparative 
anatomy  and  embryology,  from  the  geographical  distribution  of  species,  and 
from  the  geological  record.  His  presentation  is  still  one  of  the  finest  arguments 
for  evolution.  He  also  proposed  natural  selection  as  the  mechanism  making  evo- 
lution possible.  It  should  be  noted  that  evolution  could  still  be  valid  even  if,  as 
now  seems  very  unlikely,  the  theory  of  natural  selection  were  shown  to  be  false. 

The  theory  of  natural  selection  is  based  upon  a  few,  simple,  easily  veri- 
fied observations  and  the  conclusions  to  be  drawn  from  them.  It  can  readily  be 
observed  that  the  reproductive  potential  of  all  species  is  far  greater  than  is  re- 
quired to  replace  the  existing  population,  the  possible  rate  of  increase  forming  a 
geometrical  progression.  Even  elephants,  presumably  the  slowest  breeders  of  all, 
were  shown  by  Darwin  to  have  this  great  potential.  He  estimated  that  from  one 
pair,  breeding  from  age  30  to  90  and  having  only  six  young  in  this  span,  there 
would  be  descended  a  living  population  of  19,000,000  after  750  years.  The 
spread  of  the  English  sparrow  and  the  starling  after  their  introduction  into  the 
United  States  in  small  numbers  less  than  a  century  ago  is  further  evidence  of  the 
tremendous  reproductive  capacity  of  all  species,  which  is  only  realized  under  the 
most  favorable  conditions. 

Despite  this  reproductive  potential,  however,  it  can  easily  be  verified 
that  the  population  size  of  any  species  in  a  given  area  is  relatively  constant. 
Fluctuations  occur  from  year  to  year,  but  ordinarily  there  is  no  continuous 
increase. 

The  obvious  conclusion  from  these  two  observations  is  that  not  all  of 
the  progeny  produced  by  any  generation  reach  maturity,  but  that  many  die  during 
the  early  stages  of  the  life  cycle. 

The  third  observation  by  Darwin  was  that  variation  is  a  universal  phe- 
nomenon, that  no  two  individuals  are  ever  exactly  alike. 

Darwin's  final  conclusion,  then,  was  that,  since  individuals  differ  from 
each  other,  some  will  inevitably  be  better  adapted  to  survive  under  the  existing 
conditions  than  others.  Since  a  large  proportion  of  each  generation  dies  before 
reaching  maturity,  the  better  adapted  individuals  will  tend  to  survive  while  the 
less  well  adapted  will  die.  Even  though  most  of  the  deaths  occur  at  random,  if 
this  differential  affects  the  survival  of  the  remainder,  it  will  still  be  significant 
although  more  difficult  to  detect.  Finally,  if  the  adaptive  traits  are  hereditary,  the 
survivors,  who  become  the  progenitors  of  the  next  generation,  will  tend  to  trans- 
mit their  favorable  traits  to  their  offspring.  Therefore,  the  next  generation  will 
have  a  higher  proportion  of  well-adapted  individuals  than  the  previous  one. 
Hence,  in  time,  this  natural  selective  process  will  change  the  average  character- 
istics of  a  species,  and  evolution  will  occur. 


32  •  INTRODUCTION 

The  Origin  of  Species  was  widely  read  and  discussed  as  soon  as  it  ap- 
peared. Controversies  arose  over  the  validity  of  the  theories  of  evolution  and 
natural  selection.  Powerful  forces  in  the  church,  the  most  eminent  being  Bishop 
Wilberforce,  attacked  the  book,  but  there  were  also  eminent  scientists  such  as  the 
anatomist  Richard  Owen  and  the  Swiss  American  zoologist  Louis  Agassiz  who 
did  not  accept  its  conclusions.  The  distinguished  German  embryologist  von  Baer 
accepted  evolution  but  rejected  natural  selection,  for  he  did  not  accept  the  idea 
of  a  completely  materialistic  system.  The  strongest  advocate  of  Darwin's  views, 
since  his  health  limited  his  participation  in  the  public  discussions  stimulated  by 
his  book,  was  Thomas  Henry  Huxley.  In  lectures,  articles,  and  debates  Huxley 
educated  the  world  on  the  significance  of  these  theories.  The  acceptance  of 
Darwin's  views  came  quite  rapidly  among  the  scientists,  but  somewhat  more 
slowly  by  the  general  public.  Owen  rather  weakened  his  case  in  opposition  when 
it  was  discovered  that  anonymous  articles,  attacking  Darwinism  and  citing  the 
eminent  authority,  Dr.  Richard  Owen,  had  actually  been  written  by  Owen  him- 
self. The  position  of  the  church  was  modified,  in  part  at  least,  as  the  result  of 
the  famous  debate  between  Bishop  Wilberforce  and  Huxley  in  which  Huxley 
won  a  decisive  victory.  When  Bishop  Wilberforce  "begged  to  know,  was  it 
through  his  grandfather  or  his  grandmother  that  he  claimed  his  descent  from  a 
monkey?"  Huxley  replied  that  he  would  not  be  ashamed  to  have  a  monkey  for 
an  ancestor,  but  he  would  be  "ashamed  to  be  connected  with  a  man  who  used 
great  gifts  to  obscure  the  truth,"  and  with  this  stirring  statement,  he  won  the 
day.  A  rather  similar  debate  took  place  at  Harvard  between  Asa  Gray  and  Louis 
Agassiz.  With  the  support  of  such  distinguished  advocates  as  Lyell,  Huxley,  and 
Hooker  in  England,  Asa  Gray  in  America,  and  Haeckel,  an  embryologist,  and 
Gegenbauer,  a  comparative  anatomist  in  Germany,  Darwin's  theories  within  a 
very  few  years  gained  a  strong  foothold  in  the  world  of  ideas. 

The  effects  of  Darwin's  theories  on  biology  were  far  reaching.  System- 
atics  received  a  great  stimulus,  for  now  the  rationale  behind  classification  was 
the  actual  relationship  among  the  different  species.  Systematics  became  the  study 
of  evolution.  Similarly,  comparative  embryology  and  comparative  anatomy  under- 
went rapid  growth  as  their  value  in  the  working  out  of  phylogenies  became 
apparent.  Paleontology,  of  course,  as  the  main  source  of  information  about  the 
past  history  of  living  things,  also  received  a  great  impetus.  Other  fields  were 
influenced  to  greater  or  lesser  degrees,  but  none  remained  untouched. 

In  the  years  just  after  the  book  was  published,  the  major  advances  were 
made  in  disentangling  the  phylogenetic  threads.  The  theory  of  natural  selection 
was  accepted  by  the  adherents  of  Darwinism  and  condemned  by  its  opponents, 
both  without  much  evidence.  The  major  weakness  in  the  theory  of  natural  selec- 
tion was  the  lack  of  understanding  of  variation  and  its  mode  of  transmission 
from  one  generation  to  the  next.  Darwin  recognized  this  weakness  better  perhaps 
than  most  of  his  adherents.  The  basic  principles  of  heredity  were  known,  but 


DARWIN  AND  AFTER  DARWIN  •  33 

they  were  understood  apparently  only  by  their  discoverer,  Mendel;  others  who 
knew  of  his  work  either  failed  to  understand  it  or  else  failed  to  appreciate  its 
significance.  Darwin,  who  might  have  been  the  one  person  capable  of  appre- 
ciating Mendel's  work,  never  became  cognizant  of  it.  It  is  interesting  to  speculate 
what  the  course  of  events  might  have  been  if  Mendel  had  written  to  Darwin  of 
his  results.  But  it  never  happened,  and  Mendel's  work  lay  neglected  from  1865 
to  1900. 

However,  progress  was  being  made  in  still  another  area  of  biology,  the 
study  of  the  cell,  particularly  by  Strasburger  and  Flemming.  The  details  of  the 
structure  and  behavior  of  the  various  parts  of  the  cell  were  worked  out  in  the 
closing  years  of  the  nineteenth  century.  In  particular,  the  chromosomes  were 
identified,  and  the  details  of  their  behavior  during  cell  division  and  gameto- 
genesis  were  scrutinized.  Cytology  became  a  separate  branch  of  biology.  A  syn- 
thesis of  much  of  this  work  was  undertaken  by  Weismann,  who  realized  that 
the  hereditary  material  must  reside  in  the  nucleus  on  the  chromosomes,  and  who 
also  originated  the  "germ  line"  theory.  This  theory  pointed  out  that  the  germ 
cells  are  set  aside  very  early  in  development  and  are  uninfluenced  by  the  rest  of 
the  cells  in  the  body,  the  somatic  cells.  Under  this  theory,  the  inheritance  of 
acquired  characteristics  would  be  impossible.  Furthermore,  the  suggested  mech- 
anism for  such  inheritance,  Darwin's  theory  of  pangenesis,  was  outmoded.  The 
pangenes  had  been  visualized  as  being  formed  in  all  parts  of  the  body  and, 
bearing  the  traits  exhibited  there,  coming  together  to  form  the  gametes.  There  is 
no  evidence  for  this  theory  proposed  by  Darwin. 

In  1900,  Correns,  de  Vries,  and  von  Tschermak  independently  discov- 
ered Mendel's  paper,  after  essentially  reaching  Mendel's  results,  and  the  new 
science  of  genetics  finally  was  born.  Mendel's  laws  were  a  major  step  forward  in 
the  understanding  of  variation.  They  showed  that  variations  were  inherited  in  a. 
particulate  fashion,  and  that  .blending  inheritance,  visualized  by  Darwin,  did  not 
occur.  Hence,  variability  is  not  lost  in  crossing,  but  rather,  as  Hardy  and  Wein- 
berg independently  suggested  in  1908,  tends  to  remain  constant  in  a  population. 
Furthermore,  Mendel's  work  led  to  an  understanding  of  the  way  in  which  the 
recombination  of  characters  could  occur  with  the  consequent  new  variations. 

The  rise  of  genetics,  despite  its  contributions  to  the  understanding  of 
variation,  was  followed  by  a  general  eclipse  of  the  theory  of  natural  selection  as 
the  mechanism  of  evolution.  Its  place  was  taken  by  the  mutation  theory  of  de 
Vries,  proposed  in  1902.  In  his  work  with  the  evening  primrose,  Oenothera, 
de  Vries  occasionally  found  sports — that  is,  distinctly  different  types  of  plants, 
now  known  by  the  more  pedestrian  term  "mutations."  He  therefore  proposed 
that  evolution  was  not  due  to  the  gradual  accumulation  of  numerous  small 
changes  by  natural  selection,  but  instead  occurred  as  the  result  of  large  jumps 
made  possible  by  mutations  of  the  type  he  was  discovering.  This  theory  won 
wide  support  among  early  geneticists,  for  the  variations  familiar  to  them  in 


34  •  INTRODUCTION 

their  work  were  of  this  type,  and  did  not  conform  at  all  to  Darwin's  concept. 
Thus,  such  eminent  geneticists  as  William  Bateson  and  Thomas  Hunt  Morgan 
led  the  way  in  the  early  years  of  the  century  in  rejecting  natural  selection,  and 
many  others  concurred.  Ironically,  most  of  de  Vries'  mutations  were  later  demon- 
strated to  be  due  to  chromosomal  changes  rather  than  to  changes  in  the  genes 
themselves,  and  hence  were  not  mutations  in  the  usual  restricted  sense  at  all. 

Still  further  reason  to  doubt  Darwin's  theory  came  with  Johannsen's 
demonstration,  in  1910,  that  selection  was  effective  only  in  genetically  hetero- 
geneous populations  and  was  completely  without  effect  on  environmental  varia- 
tions. Darwin's  failure  to  distinguish  clearly  between  hereditary  and  environ- 
mental variation  and  his  acceptance  of  Lamarckianism  were  thus  shown  decisively 
to  be  in  error. 

Not  all  biologists  followed  the  lead  of  the  geneticists.  Many  felt,  as  the 
paleontologist  Simpson  puts  it,  "that  a  geneticist  was  a  person  who  shut  himself 
in  a  room,  pulled  down  the  shades,  watched  small  flies  disporting  themselves  in 
milk  bottles,  and  thought  that  he  was  studying  nature."  The  studies  of  the  fossil 
record  revealed,  where  the  evidence  was  complete,  that  evolutionary  changes 
had  been  gradual  rather  than  abrupt.  Taxonomists,  in  their  work  with  living 
species,  found  that  the  different  species  and  subspecies  differed  from  each  other 
in  numerous  minor  quantitative  traits  rather  than  in  a  few  major  characteristics. 
Furthermore,  a  group  of  students  of  heredity  who  worked  with  continuously 
varying  traits  rather  than  the  alternative  traits  so  commonly  studied  by  Mendel ian 
methods  obtained  results  more  in  keeping  with  Darwin's  ideas  than  those  of  the 
new  Mendelian  genetics.  This  group  had  its  origin  with  Galton,  Darwin's  first 
cousin,  well  before  1900,  and  was  responsible  for  the  development  of  the  science 
of  biometry.  Karl  Pearson  was  the  biometrician  who  came  most  directly  into  con- 
flict with  the  early  Mendelians,  led  in  England  by  Bateson.  Neither  side  recog- 
nized any  merit  in  the  work  of  the  other  group.  Feelings  ran  so  high  that 
Bateson,  in  order  to  get  his  experimental  results  into  print,  had  to  start  his  own 
journal.  However,  people  such  as  the  paleontologists,  taxonomists,  and  bio- 
metricians  who  continued  to  believe  in  natural  selection  were  frequently  regarded 
as  out-of-date  die-hards. 

A  major  advance  was  made  when  it  was  shown  that  continuous  varia- 
tion had  a  Mendelian  basis.  Thus  a  reconciliation  was  possible  between  the 
Mendelians  and  the  followers  of  Pearson.  Since  then,  there  has  come  about  a 
synthesis  leading  to  an  evolutionary  theory  that  is  now  generally  accepted  among 
paleontologists,  systematists,  geneticists,  and  most  other  biologists.  Underlying 
this  new  synthesis  is  the  increased  knowledge  and  understanding  of  variation. 
Morgan  and  his  co-workers  conclusively  demonstrated  that  the  Mendelian  factors 
or  genes  were  located  on  the  chromosomes,  and  thus  established  not  only  the 
physical  basis  of  heredity  but  of  evolution.  Our  understanding  of  the  nature  of 
mutation  and  of  the  mutation  process  has  greatly  increased,  notable  advances 


DARWIN    AND   AFTER   DARWIN  •   35 

being  Muller's  induction  of  mutations  with  x-rays  in  1927,  and  the  more  recent 
success  of  chemical  mutagens,  first  demonstrated  by  Auerbach.  The  direct  appli- 
cation of  genetic  knowledge  to  evolutionary  problems  was  made  possible  by  the 
theoretical  development  by  Fisher,  Haldane,  Tchetverikov,  and  Wright  of  popu- 
lation genetics.  As  a  result  of  their  efforts,  evolutionary  change  has  come  to  be 
recognized  as  the  result  of  the  combined  effects  of  several  forces  on  the  fre- 
quencies of  the  genes  in  breeding  populations.  One  of  these  forces  is  natural 
selection,  which  remains  as  a  cornerstone  to  an  expanded  and  strengthened 
theory  of  the  mechanism  of  evolution.  The  modern  synthesis  or  Neo-Darwinism, 
as  it  is  often  called,  has  been  largely  responsible  for  the  renewed  interest  in 
evolutionary  problems. 

SUGGESTED  READING 

See  references  at  the  end  of  Chapter  2. 


PART 


n 


The  Evidence 
for  Evolution 


CHAPTER 


4 

The  Fossil  Record 


Living  species,  by  their  very  existence,  pose  the  questions 
that  the  theory  of  evolution  attempts  to  answer,  but  the  fossil 
record  is  another  material  source  from  which  information  and  in- 
sight can  be  derived.  Few  people  have  ever  tried  to  deny  the 
existence  of  living  species,  but  many  interpretations  have  been 
made  of  the  fossils  that  have  been  found  all  over  the  world. 
These  interpretations  now  have  passed  into  the  realm  of  myths, 
and  fossils  are  generally  accepted  for  what  they  are,  the  remains  or 
traces  of  previously  existing  animals  and  plants  preserved  in  the 
earth's  crust.  The  fossil  record,  unfortunately,  is  incomplete,  but 
the  reasons  for  the  gaps  in  the  record  will  become  clear  from  a 
knowledge  of  the  nature  of  fossils  and  the  conditions  necessary 
for  their  formation.  The  two  conditions  under  which  a  fossil  is 
generally  formed  from  a  living  organism  are  that  it  have  some 
hard  parts,  and  that  it  be  buried  quickly  in  some  protecting 
medium.  Quick  burial  tends  to  retard  or  prevent  the  decomposi- 
tion of  the  organisms  by  solution  or  oxidation  or  bacterial  action. 
Fossils  have  been  formed  in  such  places  as  the  floors  of  caves,  in 
tarpits  and  oil  seeps,  in  bogs  and  quicksand,  and  under  volcanic 
ash  or  windblown  sands,  but  the  great  majority  have  been  covered 
over  by  water-borne  sediments. 

A  fossil  may  be  anything  from  an  intact  woolly  mam- 
moth frozen  in  the  Siberian  tundra  to  the  footprint  of  a  dinosaur. 
Complete  organisms,  however,  are  very  rare,  and  even  unchanged 
hard  parts,  such  as  bone,  shell,  or  woody  tissue,  are  uncommon. 
Usually  the  fossil  has  undergone  some  change,  with  the  original 


39 


40  •  THE    EVIDENCE    FOR   EVOLUTION 

hard  parts  having  gradually  been  replaced  by  some  mineral  substance  such  as 
calcium  carbonate,  silica,  or  iron  pyrite.  This  particle-by-particle  replacement  is 
so  slow  that  the  microscopic  structure  of  the  hard  parts  is  preserved,  and  the  cell 
walls  of  wood,  for  example,  can  still  be  studied  even  though  the  organic  matter 
is  completely  gone.  In  some  cases,  however,  especially  in  plants,  the  more  volatile 
elements  may  be  distilled  off,  leaving  behind  them  a  carbon  residue.  If  the 
original  hard  parts  are  dissolved,  a  "mold"  of  the  shape  may  then  be  left  in  the 
surrounding  rock.  If  the  mold  is  subsequently  filled  by  a  foreign  mineral  sub- 
stance, such  as  quartz,  a  "cast"  is  formed.  The  cast,  of  course,  retains  no  indica- 
tions of  the  original  microscopic  structure. 

The  normal  habitat  of  many  species  has  undoubtedly  precluded  their 
appearance  in  the  fossil  record  simply  because  conditions  were  unsuitable  for 
fossil  formation,  as  in  the  deep  seas  or  high  uplands,  for  example.  Even  if 
buried,  the  organism  needs  hard  parts,  for  otherwise  the  chances  of  preservation 
are  very  slight.  Whole  groups  of  species  may  be  virtually  absent  from  the  fossil 
record  because  they  did  not  meet  these  requirements.  The  fossil  record  is  there- 
fore by  no  means  a  random  sample  of  all  previously  existing  species,  but  a 
specially  selected  group.  From  the  nature  of  the  record,  it  is  obvious  that  it  will 
never  be  complete,  although  subsequent  finds  will  tend  always  to  narrow  the 
gaps  and  to  supply  the  "missing  links." 

In  addition  to  the  information  about  life  in  the  past,  fossils  reveal  still 
other  facts  about  past  conditions.  The  discovery  of  the  fossil  remains  of  marine 
organisms  like  corals  and  sea  urchins  far  inland  in  Indiana  or  20,000  feet  up  in 
the  Himalayas  has  far-reaching  geological  implications,  for  at  one  time  Indiana 
must  have  been  covered  by  the  ocean,  as  were  the  Himalayas,  which  were  subse- 
quently thrust  up  to  their  present  towering  heights.  Fossil  palms  and  alligators 
in  the  Dakotas  and  musk  oxen  in  Arkansas  are  indicative  of  wide  fluctuations  in 
past  climatic  conditions. 

Reconstructing  the  Past 

Perhaps  the  greatest  accomplishment  of  the  paleontologists  has  been 
their  reconstruction  of  the  sequence  of  past  events.  Water-borne  sediments  are 
deposited  in  layers  or  strata  that  are  then,  through  pressure,  converted  to  rock. 
Undisturbed  deposition  over  a  long  period  of  time  has  thus  given  rise  to  an 
accumulation  of  sediments  many  feet  thick,  with  the  oldest  deposits  at  the  bottom 
and  the  most  recent  at  the  top.  The  fossils  in  the  bottom  layers  must,  therefore, 
represent  the  oldest  species.  If  it  were  possible  to  find  a  place  where  deposition 
of  sediments  had  been  continuous  since  the  formation  of  the  earth  in  its  present 
structure,  the  strata  would  form  a  complete  geological  column,  and  the  included 
fossils  would  furnish  a  fairly  good  record  of  the  forms  of  life  that  had  existed 
during  this  period.  Although  some  deposits  are  thousands  of  feet  thick,  no  such 


THE    FOSSIL    RECORD  •  4l 

complete  geological  column  is  known.  Such  thicknesses,  built  up  very  gradually, 
give  some  appreciation  of  the  vastness  of  geological  time,  yet  they  represent  only 
small  fractions  of  the  total.  In  a  given  bed  of  sedimentary  rock,  the  fossils  in 
different  strata  are  different  from  one  another,  but  the  fossils  in  adjacent  layers 
are  more  alike  than  those  further  removed.  The  more  recently  formed  fossils 
show  greater  similarity  to  existing  species  than  those  in  the  lower  strata.  The 
presence  of  the  same  types  of  fossils  in  deposits  in  different  parts  of  the  world 
has  been  assumed,  as  seems  reasonable,  to  indicate  that  these  sediments  were 
laid  down  at  approximately  the  same  time.  On  this  basis,  it  has  been  possible  to 
correlate  the  deposits  all  over  the  world  into  one  chronological  series,  and  a 
geological  column  has  been  constructed  through  these  correlations.  Thus  has  the 
earth's  history  been  reconstructed.  New  finds  can  be  fitted  into  the  rest  of  the 
record,  but  the  dating  is  relative  rather  than  absolute.  The  absolute  age,  which  is 
obtained  from  studies  of  radioactive  minerals,  has  been  estimated  as  about  4.5 
billion  years.  Though  rocks  apparently  bearing  fossils  of  algae,  protozoans,  and 
fungus  spores  have  been  estimated  to  be  as  old  as  3.3  billion  years,  the  record 
was  very  fragmentary  up  until  about  500  million  years  ago.  Some  of  the  major 
subdivisions  of  geological  time  are  shown  in  Table  4-1.  Though  the  major  phyla 
have  been  represented  in  the  fossil  record  ever  since  the  Paleozoic,  the  species 
representing  each  phylum  have  changed  considerably  with  the  passage  of  time. 


TABLE    4-  1 

The  Geological  Time  Scale  (After  Kulp) 


Time  estimated  in 

Era 

Period 

Epoch 

millions  oi 

Since 
Beginning 

years 
Duration 

Cenozoic 

Quaternary 

Recent 

.011 

.011 

(Age  of  Mammals) 

Pleistocene 

1 

1 

Tertiary 

Pliocene 

13 

12 

Miocene 

25 

12 

Oligocene 

36 

11 

Eocene 

58 

22 

Paleocene 

63 

5 

Mesozoic 

Cretaceous 

135 

72 

(Age  of  Reptiles) 

Jurassic 

181 

46 

Triassic 

230 

49 

Paleozoic 

Permian 

280 

50 

(Age  of  Fishes) 

Pennsylvanian 

310 

30 

Mississippian 

i 

345 

35 

Devonian 

405 

60 

Silurian 

42  5 

20 

Ordovician 

500 

75 

Cambrian 

600 

100 

Precambrian 

5,000± 

4,400± 

42  •  THE    EVIDENCE    FOR   EVOLUTION 

The  Paleozoic,  for  example,  is  known  as  the  Age  of  Fishes,  the  Mesozoic  as  the 
Age  of  Reptiles,  and  the  Cenozoic  as  the  Age  of  Mammals;  the  mammals  first 
appeared  in  the  fossil  record  during  the  late  Mesozoic  but  reached  their  climax 
only  during  the  Cenozoic. 

Even  as  recently  as  the  Mesozoic  era,  practically  no  living  species  ex- 
isted. Many  species  have  appeared  in  the  fossil  record,  persisted  in  it  for  varying 
periods,  and  then  disappeared.  Where  the  record  is  fairly  complete,  gradual 
changes  within  a  given  group  can  be  followed  from  the  older  to  the  more 
recent  strata.  The  evidence  shows  that  distinct  new  species  have  appeared  in  all 
parts  of  the  world  throughout  geological  time.  There  is  no  time  or  place,  ap- 
parently, at  which  new  species  could  not  have  originated.  The  most  reasonable 
and  complete  explanation  for  the  evidence  from  the  rocks — physical  evidence 
that  can  hardly  be  ignored — is  the  theory  of  evolution;  that  is,  living  species, 
through  a  series  of  gradual  changes,  have  descended  from  somewhat  different 
species  living  in  the  past.  Today,  in  fact,  we  think  of  the  fossil  record  in  terms 
of  evolution  to  such  a  degree  that  it  is  hard  to  separate  the  record  from  its  inter- 
pretation. Yet  Cuvier,  in  Lamarck's  time,  and  Louis  Agassiz  in  Darwin's,  prob- 
ably the  leading  paleontologists  of  their  day,  both  opposed  the  theory  of  evolu- 
tion, using  the  paleontological  materials  to  support  their  arguments.  Since  then, 
however,  our  vastly  increased  knowledge  about  paleontology,  due  in  large  part 
to  the  stimulus  of  Darwin's  theories,  has  made  it  one  of  the  bulwarks  of  proof 
that  evolution  has  actually  occurred. 

Extinction  and  Evolution 

Practically  all  of  the  species  recognized  from  fossils  no  longer  exist. 
There  are  two  routes  to  extinction — one  leading  to  complete  extinction;  the 
other,  through  evolutionary  change,  to  new  species.  The  evolution  of  new  species 
may  take  place  in  two  ways.  One  is  a  transformation  in  time,  species  A  evolving 
into  B,  B  into  C,  and  so  on  as  time  passes.  The  other  is  a  multiplication  of 
species  in  space,  two  species,  B  and  C,  originating  simultaneously  from  a  single 
species,  A.  Because  of  this  latter  process  (now  usually  referred  to  as  speciation, 
in  a  restricted  sense  of  the  word),  the  number  of  coexisting  species  has  tended 
to  increase  as  more  and  more  of  the  available  ecological  niches  have  been  occu- 
pied. For  example,  invasion  of  the  land  did  not  occur  until  plant  and  animal 
species  adapted  to  life  on  land  had  evolved  from  the  ancient  aquatic  types.  A 
whole  new  range  of  possibilities  then  opened  up,  and  adaptive  radiation  of 
species  from  these  first  successful  invaders  of  the  land  into  a  variety  of  diverse 
habitats  occurred.  Because  the  process  of  adaptive  radiation  through  speciation 
has  continued  through  geological  time,  the  number  of  living  species  is  probably 
greater  today  than  it  has  been  at  any  time  in  the  past. 

Evolutionary  changes  are  gradual,  with  no  positive  evidence  for  the 


THE    FOSSIL   RECORD  •  43 

formation  of  species  by  a  cataclysmic  process  or  saltation  existing  in  the  fossil 
record,  but  all  evolutionary  rates  are  not  the  same.  Different  groups  may  have 
different  average  rates  of  evolution;  the  mammals,  for  example,  appear  to  have 
evolved  much  more  rapidly  than  the  ammonites.  Even  within  a  single  group,  the 
rate  of  evolution  may  change  from  one  time  to  another.  Though  evolution  goes 
on  between  generations  rather  than  within  generations,  nevertheless  generation 
length  seems  to  have  no  relation  to  evolutionary  rates,  for  the  mammals,  with  a 
very  long  generation  length,  have  had  an  extremely  rapid  rate  of  evolution. 

Frequently,  the  slow,  steady  evolution  within  a  particular  evolutionary 
line  shows  a  series  of  changes  in  a  single  direction  or  a  trend,  a  type  of  evolu- 
tion known  as  "orthogenesis."  Because  such  trends  are  so  common,  it  has  been 
suggested  that  evolution  may  have  a  sort  of  momentum,  which  causes  it,  once 
under  way,  to  continue  to  move  in  the  same  direction,  even  when  the  changes 
are  no  longer  adaptive.  It  has  been  suggested,  for  example,  that  the  Irish  elk 
became  extinct  when  its  massive  antlers  became  so  heavy  that  the  animals  could 
no  longer  hold  up  their  heads  or  else  snagged  them  in  the  brush  and  thus  starved 
to  death.  The  saber-toothed  tiger  was  supposed  to  have  met  a  similar  fate  when 
his  fearsome  fangs  became  so  long  that  he  could  no  longer  get  any  food  past 
them.  However,  more  thorough  study  has  shown  that  the  trends  are  due  to  con- 
stant selection  pressure  in  a  given  direction,  and  that  the  changes  are  adaptive; 
hence  the  term  "orthoselection"  would  be  more  descriptive  than  orthogenesis. 
Whatever  the  causes  of  extinction  for  the  Irish  elk  and  the  saber-toothed  tiger, 
they  were  not  carried  off  by  runaway  evolution. 

One  implication  of  orthogenesis,  divorced  as  it  is  from  adaptation,  is 
that  there  is  a  vital  force  or  elan  vital  animating  all  living  things.  In  addition, 
the  prevalence  of  evolutionary  trends  has  led  to  speculation  that  evolution  is 
directed  toward  some  ultimate  goal,  a  concept  known  as  "finalism."  There  is  no 
reason  or  need,  however,  to  invoke  either  vitalism  or  finalism  to  account  for  the 
evidence. 

Major  adaptive  shifts,  giving  rise  to  new  and  distinctive  groups,  repre- 
sent changes  in  the  direction  of  evolution  and  usually  a  change  in  rate  as  well. 
The  gaps  in  the  fossil  record  usually  seem  to  occur  at  the  crucial  stages  where, 
if  evolution  is  a  gradual  process,  transitional  forms  connecting  major  groups 
ought  to  be  found.  Failure  to  find  many  transitional  fossils  has  led  many  author- 
ities to  postulate  a  different  evolutionary  mechanism  for  the  origin  of  higher 
taxonomic  groups,  but  our  subsequent  discussion  will  show  that  no  special 
mechanism  is  demanded  by  the  evidence. 

In  discussions  of  trends  in  evolution,  the  terms  "generalized"  and  "spe- 
cialized" are  frequently  used,  often  with  the  corollary  that  "specialization  is  the 
prelude  to  extinction."  Such  a  generalization  is  unwarranted.  The  terms  "gen- 
eralized" and  "specialized"  have  meaning  only  in  a  relative  and  rather  limited 
sense,  though  they  can  be  useful.  To  raise  a  specific  question,  were  the  early 


44  •  THE    EVIDENCE    FOR   EVOLUTION 

mammals  specialized  or  generalized?  Had  a  zoologist  of  the  day  (if  such  existed) 
compared  them  with  their  contemporaries,  the  dominant  reptilian  group,  they 
might  well  have  been  considered  a  small,  specialized,  and  rather  aberrant  group 
of  reptiles,  destined  therefore  to  rapid  extinction.  In  this  instance  specialization 
was  a  prelude  to  new  evolutionary  opportunities.  Compared  with  recent  mam- 
mals, however,  these  early  mammals  must  be  considered  quite  generalized.  A 
rather  similar  verbal  pitfall  is  found  in  the  use  of  the  terms  "primitive"  and 
"modern"  species.  The  shark  and  the  frog,  for  instance,  are  often  cited  as  ex- 
amples of  primitive  vertebrates,  with  the  mammals  held  up  as  the  modern  type. 
Since  sharks,  frogs,  and  mammals  are  all  living  today,  one  group  is  just  as  old  as 
the  other,  and  the  ancestry  of  one  can  be  traced  back  just  as  far  as  that  of 
another,  though  a  greater  variety  of  ancestors  may  appear  in  one  lineage.  The 
fallacy  would  be  even  clearer  if,  through  some  quirk  of  fate,  all  mammals  be- 
came extinct.  If  used  with  reference  to  time  of  origin,  however,  the  terms  can 
be  useful  and  not  especially  confusing. 

Vertebrate  Evolution 

In  order  to  give  some  appreciation  of  the  type  of  information  available 
in  the  fossil  record,  the  history  of  the  vertebrates  or  backboned  animals  (the 
subphylum  Vertebrata  of  the  phylum  Chordata)  will  be  outlined  (see  Fig.  4-1). 
The  first  vertebrate  fossils  appeared  in  the  Ordovician  period  of  the  Paleozoic 
era,  which  began  about  425  million  years  ago.  These  fishlike  animals  were  small, 
armored,  bottom  dwellers,  but  lacked  both  jaws  and  paired  fins.  Known  as 
ostracoderms,  they  belonged  to  the  class  Agnatha,  which  today  is  represented  by 
just  a  few  surviving  species,  the  most  familiar  being  the  lampreys.  The  Agnatha 
remained  common  throughout  the  Silurian  and  Devonian  periods.  The  first 
vertebrates  to  have  jaws  and  paired  appendages  appeared  among  the  late  Silurian 
fossils,  were  very  common  in  the  Devonian  (325  million  years  ago),  and  had 
virtually  disappeared  from  the  Mississippian  record.  This  class  of  early  fishes,  the 
Placodermi,  is  now  extinct.  The  Chondrichthyes,  a  group  to  which  the  present- 
day  sharks  and  rays  belong,  first  appeared  in  the  middle  and  late  Devonian, 
became  abundant  in  the  Mississippian  and  Pennsylvanian,  and  have  remained 
common  up  to  the  present  day.  At  about  the  same  time  the  bony  fishes 
(Osteichthyes)  appeared  in  the  fossil  record  and  have  flourished  ever  since. 
Unlike  the  sharks,  they  had  a  specialized  spiracle,  an  added  (hyoid)  support  for 
the  jaws,  and  an  air  bladder  or  lungs.  They  include  two  major  groups,  the 
Choanichthyes,  including  the  lobe-finned  fishes  or  crossopterygians  and  the  living 
lung  fishes  or  Dipnoi,  and  the  Actinopterygii,  or  ray-finned  fishes,  to  which 
belong  more  than  90  percent  of  the  existing  species  of  fish. 

The  first  land  vertebrates,  with  legs  and  lungs,  did  not  appear  as  fossils 
until  the  late  Devonian.  These  first  tetrapods  were  amphibians,  a  group  that  had 


THE    FOSSIL   RECORD  •  45 


Periods 
Cretaceous 

Jurassic 

Triassic 
Permian 

Pennsylvanian 

» 

Mississippian 

Devonian 

Silurian 


PLACODERMI  / 


Ordovician 


Fig.  4-1.     The  phylogeny  of  the  vertebrates.    (After  Romer). 

its  heyday  during  the  Mississippian  and  Pennsylvanian  periods  and  has  since 
been  a  subordinate  part  of  the  land  vertebrate  fauna,  represented  today  by  the 
frogs,  toads,  and  salamanders.  The  first  known  reptiles  were  found  in  rocks  of 
Pennsylvanian  origin.  Though  it  is  a  relatively  simple  matter  to  distinguish  be- 
tween recent  amphibians  and  reptiles,  the  criteria  tend  to  break  down  for  the 
ancient  species.  One  reason  for  this  difficulty  is  that  the  most  significant  differ- 
ence between  the  amphibians  and  reptiles  lies  in  their  modes  of  reproduction  and 
development.    The   reptiles   were   completely   freed   from   dependence   on    an 


46  •  THE    EVIDENCE    FOR   EVOLUTION 

aquatic  environment  at  any  stage  of  their  life  cycle  because  their  shelled  eggs 
could  develop  on  land.  The  amphibian  egg,  with  little  yolk,  must  be  laid  in  the 
water,  and  the  young  tadpoles,  a  larval  stage,  soon  emerge.  The  developing 
embryo  of  the  reptilian  egg  is  bathed  in  fluid,  too,  but  the  fluid  is  contained  in 
a  sac,  the  amnion,  which  encloses  the  embryo.  Another  membranous  sac,  the 
allantois,  serves  as  a  respiratory  structure  for  gaseous  exchange  and  also  as  a 
storage  place  for  excretory  wastes.  Because  of  the  large  yolk  supply,  the  young 
reptiles  develop  much  further  than  the  amphibians  before  they  hatch  from  the 
egg.  The  reptiles  increased  in  numbers  during  the  Permian  and  were  the  domi- 
nant land  vertebrates  throughout  the  Mesozoic  era.  Many  of  the  reptilian  groups 
then  prominent,  such  as  the  dinosaurs,  ichthyosaurs,  mosasaurs,  and  plesiosaurs, 
are  now  extinct,  and  the  reptiles  today  are  represented  by  such  groups  as  the 
snakes,  turtles,  alligators,  and  lizards. 

The  first  birds  (Aves)  appeared  in  the  fossil  record  in  the  Jurassic 
period  of  the  Mesozoic,  but  unlike  modern  birds,  which  did  not  appear  until 
the  Cenozoic,  they  had  teeth  and  a  tail  composed  of  vertebrae,  and  were  difficult 
to  distinguish  from  reptiles.  Even  today  birds  seem  much  like  glorified  reptiles. 

Though  mammallike  reptiles  (Therapsids)  existed  in  the  late  Paleozoic, 
the  first  true  mammals  did  not  appear  as  fossils  until  the  Triassic  and  they  did 
not  form  an  important  part  of  the  fauna  until  the  Cenozoic.  The  mammals  are 
characterized  by  the  presence  of  mammary  glands,  hair,  warm  blood,  and  a  rela- 
tively large  brain,  which  is  probably  in  large  measure  responsible  for  their  cur- 
rent dominance.  Though  most  mammals  bear  living  young,  which  have  under- 
gone development  in  the  uterus  of  the  mother  while  nourished  via  the  placenta, 
some  living  mammals,  such  as  the  duck-billed  platypus,  lay  shelled  eggs.  This 
group,  the  monotremes  or  Protheria,  is  apparently  quite  distantly  related  to  the 
mammalian  lines  of  descent  that  gave  rise  to  the  marsupials  (Metatheria)  and 
the  placental  mammals  (Eutheria).  The  first  fossils  that  show  clearly  human 
affinities  appeared  in  the  fossil  record  less  than  two  million  years  ago. 

Evolution  of  the  Horse 

Horses  have  left  behind  the  most  complete  sequence  of  fossils  yet  dis- 
covered. Their  history  has  therefore  been  worked  out  in  greater  detail  than  that 
of  any  other  group  (see  Fig.  4-2).  Man  and  the  horse  have  been  closely  asso- 
ciated for  centuries,  but  whereas  the  human  fossil  record  has  been  traced  back 
to  something  less  than  two  million  years,  fossil  horses  first  make  an  appearance 
in  the  early  Eocene  some  sixty  million  years  ago.  Although  no  direct  links  with 
animals  living  in  the  Paleocene  are  known,  the  indications  are  that  the  horses,  or 
Equidae,  are  descended  from  the  order  Condylarthra,  an  order  of  five-toed 
hoofed  mammals  or  ungulates  that  is  now  extinct.  Horses  belong  to  the  order  of 
odd-toed  ungulates,  the  Perissodactyla,  and  number  among  their  relatives  the 


THE    FOSSIL   RECORD  •  47 


SOUTH    AMERICA 


NORTH     AMERICA 


OLO    WORLD 


Fig.  4-2.     The  evolution  of  the  horse.    (With  permission  of  Simpson.) 


living  rhinoceroses  and  tapirs  and  the  extinct  chalicotheres  with  clawed  feet  and 
the  enormous  "horned"  brontotheres. 

The  primary  center  for  horse  evolution  was  in  North  America,  especially 
in  the  Great  Plains  region,  for  the  most  abundant  and  continuous  fossil  record 
has  been  found  there.  From  time  to  time  some  of  the  species  spread  to  the  Old 
World,  but  not  until  a  land  connection  was  again  established  at  the  end  of  the 
Pliocene  were  they  able  to  reach  South  America.  Consequently  the  fossil  record 
of  the  horses  on  that  continent  is  confined  to  Pleistocene  deposits. 


48  •  THE    EVIDENCE    FOR    EVOLUTION 

The  earliest  Eocene  equines  were  so  unlike  the  modern  horses  that  they 
were  called  Hyracotherium  because  of  their  rodentlike  appearance.  They  later 
became  known  as  eohippus,  the  dawn  horse.  Eohippus,  from  which  all  subse- 
quent horse  evolution  proceeded,  was  a  small,  browsing  animal  the  size  of  a  fox 
terrier  and  standing  only  ten  to  twenty  inches  tall  at  the  shoulder.  His  back  was 
arched,  and  his  hind  legs  and  tail  were  relatively  long.  His  front  feet  each  had 
four  toes,  the  hind  feet  only  three,  and  even  though  tiny  hoofs  were  present, 
most  of  the  weight  was  born  by  pads. 

Miohippus  from  the  Oligocene  was  the  first  horse  with  three  toes  on  all 
feet,  but  the  lateral  toes  were  still  functional.  About  the  size  of  a  sheep,  this 
browsing  horse  was  apparently  more  intelligent  and  fleet  of  foot  than  its 
predecessors. 

The  fossils  of  Merychippus  come  primarily  from  the  Miocene.  This 
group  of  horses  had  high-crowned  teeth,  adapted  for  grazing  on  the  relatively 
harsh  grasses  of  an  open  prairie  habitat,  rather  than  the  low-crowned  teeth  of  its 
predecessors,  which  were  adapted  to  browsing  on  succulent  shoots  and  leaves. 
Furthermore,  though  Merychippus  still  had  three  toes,  the  outer  toes  were  re- 
duced, barely  touching  the  ground,  and  the  leg  had  become,  with  its  well- 
developed  cannon  bone,  an  efficient  spring  mechanism.  This  group,  which 
marked  the  completion  of  the  transition  from  browsing  to  grazing,  was  highly 
successful,  numerous,  and  widespread.  Pliohippus,  the  first  one-toed  horse,  ini- 
tially appeared  in  the  Pliocene  deposits.  The  two  slender  splint  bones  on  each  side 
of  the  cannon  bone  are  the  only  vestiges  of  the  other  two  toes.  Pliohippus  was 
succeeded  in  the  Pleistocene  by  members  of  the  genus  Equus  to  which  belong 
all  the  living  Equidae — the  horses,  zebras,  asses,  and  onagers. 

In  outline,  the  material  just  presented  indicates  the  line  of  succession 
from  the  earliest  known  equids  up  to  the  present-day  horses.  The  abundant 
fossils  have  made  it  possible  to  document  the  changes  rather  than  having  to 
attempt  to  fill  gaps  in  the  record  by  speculation  or  conjecture.  The  major 
changes  from  Hyracotherium  to  Equus  were  an  increase  in  overall  size,  a  reduc- 
tion in  the  number  of  toes,  a  transition  from  browsing  to  grazing,  and  the  asso- 
ciated increase  in  the  height  and  complexity  of  the  teeth. 

To  present  this  record  without  additional  information,  however,  is  to 
give  a  greatly  oversimplified  conception  of  how  evolution  actually  took  place  in 
the  horse.  As  presented,  it  appears  to  have  been  a  linear  process,  perhaps  with 
overtones  of  orthogenesis.  Actually,  this  was  far  from  the  case.  At  each  level 
from  eohippus  on,  an  adaptive  radiation  took  place  and  numerous  groups 
evolved,  all  of  which  except  Equus  are  now  extinct.  Miohippus,  for  example, 
was  ancestral  not  only  to  Merychippus,  which  completed  the  transition  to  graz- 
ing, but  also  to  a  line  that  culminated  in  the  large  three-toed  browsing  "forest" 
horses  known  as  Hypohippus,  and  to  at  least  three  other  distinct  lineages. 
Similarly,  Merychippus,  successfully  adapted  to  grazing,  became  the  source  of  a 


THE    FOSSIL    RECORD  •  49 

number  of  three-toed  grazing  horses  such  as  the  highly  successful  genus  Hip- 
parion,  as  well  as  of  the  one-toed  group  Pliohippus.  Pliohippus  gave  rise  not 
only  to  Equus  but  also  to  the  genus  Hippidion,  which  reached  South  America  in 
the  early  Pleistocene  and  there  underwent  adaptive  radiation.  Therefore,  before 
reading  any  trends  into  the  record,  we  must  try  to  see  whether  they  really  exist. 
For  example,  horses  in  some  cases  did  increase  in  size,  but  some  lines  remained 
essentially  unchanged  for  long  periods,  and  in  others  actual  decreases  in  size 
occurred.  The  reduction  in  numbers  of  toes  was  by  no  means  universal  nor  was 
it  a  gradual,  inexorable  process.  The  change  from  four  front  toes  to  three  oc- 
curred in  a  relatively  short  period  and  was  followed  much  later  by  the  rapid 
transition  from  three  toes  to  one.  In  each  case  it  was  an  adaptive  shift  occurring 
in  one  among  a  number  of  existing  groups.  Finally,  the  change  from  low- 
crowned  to  high-crowned,  more  complex  teeth  was  one  adaptive  shift  in  the 
evolution  of  the  browsing  horses  that  happened  to  be  highly  successful  because 
it  opened  up  a  new  ecological  niche  to  exploitation.  However,  other  types  of 
trends  can  also  be  traced  in  the  evolution  of  the  teeth  of  browsing  horses.  Thus, 
this  brief  resume  indicates  that  both  the  rate  and  the  direction  of  evolution  may 
change  and  that  the  changes  seem  to  be  related  to  adaptation.  Only  so  long  as 
an  evolutionary  shift  continues  to  bring  improved  adaptation  will  it  continue. 
To  this  extent,  evolutionary  trends  may  be  observed,  but  they  are  due  to  natural 
selection,  not  to  orthogenesis  impelled  by  some  mysterious  internal  force.  The 
most  persistent  trends  would  be  expected  in  the  improvement  of  those  traits 
that  confer  adaptive  advantage  in  any  kind  of  environment. 

Several  obvious  facts  stand  out  from  this  brief  review  of  the  verte- 
brates' history.  Not  all  of  the  major  groups  of  vertebrates  have  been  represented 
since  the  Ordovician;  instead,  new  groups  have  appeared  periodically.  The  more 
recent  deposits  contain  vertebrates  much  more  like  living  species  than  the  most 
ancient  fossils.  Great  numbers  of  species  found  as  fossils  have  become  extinct. 
Though  gaps  exist  in  the  record,  types  intermediate  between  the  major  groups 
have  been  discovered.  The  most  far-reaching  and  consistent  explanation  of  the 
vast  array  of  facts  accumulated  from  the  study  of  paleontology  is  the  theory  of 
evolution.  The  sequence  of  appearance  in  the  rock  strata  depicts  the  phylogeny 
of  the  group  (see  Fig.  4-1).  A  major  advance  in  the  course  of  vertebrate  evolu- 
tion and  hence  of  human  evolution — for  man  fits  into  the  overall  scheme — was 
the  acquisition  of  jaws  and  paired  appendages  by  the  Placodermi;  another  such 
advance  occurred  when  the  lobe-finned  fishes  gave  rise  to  the  four-footed 
amphibians,  which  breathed  air  with  lungs  derived  from  the  air  sacs  or  lungs  of 
the  fishes.  Man  and  the  dog  and  the  horse  show  so  many  similarities — that  com- 
plex of  traits  characteristic  of  placental  mammals — because  they  had  a  common 
ancestry  up  until  about  75  to  100  million  years  ago. 

Although  speculation  as  to  what  follows  in  vertebrate  evolution  is  the 
next  logical  topic,  we  shall  defer  it  until  after  our  discussion  of  evolutionary 


50  •  THE    EVIDENCE    FOR   EVOLUTION 

mechanisms.  Our  purpose  now  is  to  present  the  evidence  that  evolution  has 
occurred  in  the  past,  and  of  this  evidence,  fossils  constitute  the  major  portion. 


SUMMARY  < 


The  fossil  remains  of  animals  and  plants  are  widely  dis- 
tributed over  the  earth.  Absolute  and  relative  dating  methods 
show  them  to  be  of  varying  ages — some  quite  recent,  others  of 
great  antiquity.  These  fossils  constitute  an  actual  record  of  the 
organisms  that  lived  on  the  earth  at  different  times  in  the  past. 
An  examination  of  this  record  shows  that  the  kinds  of  living  ani- 
mals and  plants  changed  gradually  with  time.  Thus,  species  ad- 
jacent in  time  are  more  alike  than  species  separated  by  vast  time 
spans,  and  the  more  recent  the  fossils,  the  more  they  tend  to  re- 
semble living  species.  The  theory  of  evolution,  of  descent  with 
modification,  provides  the  most  logical  explanation  for  the  fossil 
record.  The  living  species  of  the  past,  forced  to  adapt  to  an  ever- 
changing  physical  and  biological  environment,  underwent  gradual 
modifications  through  time.  Many  groups,  unable  to  adapt,  be- 
came extinct;  others,  more  successful,  survived  and  spread,  only 
to  be  supplanted  in  turn  by  still  better  adapted  types.  These  suc- 
cessful groups,  however,  did  not  arise  de  novo,  but  were  de- 
scended from  previously  existing  species  of  animals  and  plants. 

SUGGESTED  READING 

Colbert,  E.  H.,  1955.  Evolution  of  the  vertebrates.  New  York:  Wiley. 

Flint,  R.  F.,  1957.  Glacial  and  Pleistocene  geology.  New  York:  Wiley. 

Moore,  R.  C,  1958.  Introduction  to  historical  geology,  2d  ed.  New  York:  McGraw- 
Hill. 

Newell,  N.  D.,  1959.  "The  nature  of  the  fossil  record,"  Proc.  Amer.  Phil.  Soc. 
103:264-285. 

Romer,  A.  S.,  1945.  Vertebrate  paleontology,  2d  ed.  Chicago:  University  of  Chicago 
Press. 

,  1958.  The  vertebrate  story.  Chicago:  University  of  Chicago  Press. 

Simpson,  G.  G.,  1950.  The  meaning  of  evolution.  New  Haven:  Yale  University 
Press.  (New  York:  Mentor  Books,  1951.) 

,  1951.  Horses.  New  York:  Oxford  University  Press. 

,  1953.  Life  of  the  past.  New  Haven:  Yale  University  Press. 

,   1953.  The  major  features  of  evolution.  New  York:  Columbia  University 

Press. 

Stirton,  R.  A.,  1959.  Time,  life,  and  man.  The  fossil  record.  New  York:  Wiley. 


CHAPTER 


The  Origin  of  the  Earth 

and  of  the  Universe 


Once  it  is  known  that  the  first  fossils  are  several  hundred 
million  or  a  few  billion  years  old,  the  next  question  inevitably 
concerns  the  origin  of  life  and,  beyond  that,  the  origin  of  the 
earth  and  of  the  universe  itself.  Though  cosmogony  is  currently 
making  great  strides,  the  answers  to  these  questions  are  more 
speculative  than  those  about  the  less  remote  events  detailed  in 
the  fossil  record.  Nevertheless,  a  brief  review  of  current  thought 
on  these  questions  is  certainly  worthwhile,  as  long  as  it  is  realized 
that  this  sort  of  information  has  a  different  basis  and  hence  is 
less  reliable  than  the  reconstruction  of  past  events  based  on  actual 
fossil  remains.  The  theories  in  these  areas  are  much  more  likely 
to  change  as  new  information  becomes  available. 

On  the  basis  of  narratives  in  the  Old  Testament,  Arch- 
bishop Ussher  in  the  seventeenth  century  calculated  that  the 
world  was  created  in  4004  B.C.  The  delvers  into  such  mysteries 
among  the  people  of  ancient  India  arrived  at  a  date  that  would 
in  1962  make  the  world  1,972,949,063  years  old.  Modern  esti- 
mates, which  do  not  claim  such  precision,  generally  agree  that  the 
zero  hour  of  the  universe,  as  we  know  it,  was  a  few  billion  years 
ago. 

Age  of  the  Universe 

Science  has  used  several  approaches  to  estimate  the  age 
of  the  earth.  One  of  these  is  a  method  that  determines  the  age  of 
the  oceans.  About  3  percent  of  sea  water  consists  of  dissolved  salts. 


51 


52  •  THE    EVIDENCE    FOR   EVOLUTION 

These  salts  are  constantly  being  leached  from  the  rocks  forming  the  earth's  crust 
and  are  carried  to  the  oceans  by  the  rivers.  The  water  evaporates  from  the  surface 
of  the  oceans,  falls  on  the  land,  and  again  flows  to  the  sea  in  an  ever-renewed 
cycle,  but  the  salts  remain  in  the  sea,  the  salinity  gradually  increasing  as  time 
passes.  Each  year  about  400  million  tons  of  salt  are  added  to  the  40  X  1015  tons 
already  present  in  the  seas.  Simple  division  indicates  that  the  process  has  lasted 
for  at  least  100  million  years.  However,  since  the  rate  of  erosion  is  now  un- 
usually high  compared  to  other  periods  of  geological  time,  because  of  the  higher 
mountain  ranges  and  man's  activities,  this  estimate  must  be  increased  at  least  20 
to  30  times,  which  leads  to  an  age  of  2  or  3  billion  years.  The  very  fact  that  the 
oceans  are  not  saturated  with  salt  indicates  their  limited  existence. 

The  age  of  the  continents  can  be  determined  by  estimating  the  age  of  the 
rocks  composing  them.  The  radioactive  elements  uranium  and  thorium  are  found 
in  small  quantities  in  many  rocks,  where  both  slowly  decay  into  lead.  Once  the 
rock  has  solidified,  the  radiogenic  lead  cannot  escape,  but  remains  trapped  in 
the  rock  with  the  original  radioactive  substances.  The  uranium/lead  and 
thorium/lead  ratios  give  a  rather  exact  figure  for  the  age  of  a  given  rock  in 
much  the  same  way  an  hour  glass  might  if  each  grain  as  it  fell  were  changed  to 
lead.  Different  rocks  give  different  ages,  but  the  maximum  estimate  thus  far  is 
about  3  billion  years.  This  value  is  fairly  reliable,  but  must  be  regarded  as  the 
lower  limit  of  the  age  of  the  earth,  for  the  earth  may  well  have  been  formed 
long  before  these  rocks  solidified  into  their  present  structure.  Similar  types  of 
analyses  have  been  run  on  meteorites  in  an  effort  to  estimate  the  age  of  the  solar 
system.  The  age  of  the  meterorites  was  found  to  be  on  the  order  of  4.5  billion 
years;  the  earth,  as  a  part  of  the  solar  system,  must  also  be  approximately  of  this 
age.  Still  another  possible  type  of  analysis  is  the  determination  of  the  age  of  the 
chemical  elements  themselves — that  is,  the  matter  that  forms  the  solar  system. 
These  elements  must  have  a  finite  age;  otherwise,  by  now  the  radioactive  ele- 
ments would  have  disintegrated  and  disappeared.  Estimates  of  their  age  range 
up  to  6  billion  years. 

Astronomers  have  tackled  the  age  of  the  universe  in  several  ways.  One 
method  is  to  study  stellar  velocities  within  the  Milky  Way,  the  galaxy  of  which 
we  are  a  part.  When  such  a  system  has  existed  for  a  long  time,  the  stellar 
velocities  are  expected  to  approach  a  limiting  distribution  with  an  equal  partition 
of  energy  among  all  the  stars.  However,  this  distribution  has  not  yet  been  real- 
ized, and  the  calculations  indicate  that  the  system  has  existed  only  a  few  billion 
years. 

A  second  method  is  based  on  the  rate  at  which  a  star  burns  up.  Stars 
obtain  their  energy  from  the  nuclear  transformation  of  hydrogen  into  helium  in 
their  hot  centers.  Thus,  the  life  span  of  a  star  is  determined  by  its  brightness  or 
rate  of  burning  and  by  its  original  hydrogen  content.  Larger  stars  burn  out  faster 
than  the  smaller  ones  such  as  the  sun.  The  large  stars  that  must  have  been 


ORIGIN    OF    EARTH    AND    UNIVERSE  •   53 

formed  several  billion  years  ago  are  now  in  their  death  throes,  pulsating  and 
exploding,  but  the  smaller  sun  has  at  least  5  billion  years  to  go  before  reaching 
this  stage. 

A  third  method  of  estimating  the  age  of  the  universe  is  based  on  what 
is  called  the  "red  shift."  The  Milky  Way,  containing  billions  of  stars,  is  just  one 
of  about  a  billion  such  stellar  systems  or  galaxies  within  the  range  of  the  200- 
inch  telescope  at  Mount  Palomar,  California.  A  peculiar  feature  about  the  dis- 
tant galaxies  is  that  the  light  from  them,  although  similar  to  that  from  nearer 
ones,  shows  a  peculiar  shift  of  the  spectral  lines  toward  the  red  end  of  the 
spectrum.  A  simple  physical  explanation  for  this  shift  is  that  the  galaxies  are 
receding  at  high  speeds,  and  hence  the  universe  is  expanding.  The  effect  is 
similar  to  the  apparent  change  in  pitch  of  a  train  whistle  as  it  approaches  and 
then  recedes  from  a  crossing.  Since  this  phenomenon,  known  as  the  Doppler 
effect,  has  also  been  reported  in  the  rather  new  field  of  radio  astronomy,  both 
light  and  radio  waves  appear  to  be  similarly  affected. 

This  discovery  led  to  still  another  method  of  estimating  the  age  of  the 
universe,  on  the  assumption  that  the  universe  as  we  know  it  today  arose  as  the 
result  of  the  differentiation  of  some  sort  of  rapidly  expanding  primordial  matter. 
A  date  for  the  beginning  of  this  expansion  can  be  obtained  by  dividing  the 
average  distance  between  neighboring  galaxies  by  the  velocity  of  their  recession. 
The  original  estimate  by  this  method,  1.8  billion  years,  presented  a  puzzle  be- 
cause the  geological  estimates  already  were  much  greater  than  this.  Recently, 
however,  corrections  in  this  method  have  led  to  estimates  for  the  age  of  the  uni- 
verse as  high  as  7  to  10  billion  years.  Although  some  differences  exist  in  the 
various  estimates,  they  are  not  too  important  for  our  purposes.  Trie  age  of  the 
universe,  as  derived  from  several  independent  estimates,  seems  to  be  about 
5  billion  years  or  more. 

Nature  of  the  Universe 

The  findings  of  the  astronomers  have  led  modern  cosmologists  to  two 
quite  different  conceptions  of  the  nature  of  the  universe.  One  is  that  of  an 
evolving  universe,  the  other  a  steady-state  universe.  Under  the  evolutionary 
theory  the  expansion  indicated  by  the  red  shift  is  interpreted  to  mean  that  the 
universe  started  off  with  a  "big  bang."  The  matter  within  the  universe  was 
squeezed  together  so  tightly  and  at  such  high  temperature  and  density  that  it 
consisted  only  of  protons,  neutrons,  and  electrons,  which  did  not  form  any 
larger  elements.  When,  because  of  expansion,  the  temperature  dropped,  the 
neutrons  started  to  decay  to  protons,  and  the  neutrons  and  protons  started  to 
form  aggregations  of  atomic  nuclei.  The  rate  of  expansion  determined  the  types 
of  atoms  formed.  Physicists  have  calculated  that  the  "cooking  period"  could  not 
have  lasted  more  than  half  an  hour.  If  it  had  been  less  (a  rapid  expansion),  the 


54  •  THE    EVIDENCE    FOR   EVOLUTION 


Fig.  5-1.     The  origin  and  evolution  of  the  earth. 


universe  would  contain  mostly  hydrogen;  if  longer,  heavy  elements  would 
predominate. 

For  the  next  250  million  years,  radiant  energy  was  predominant  over 
matter  (the  two  being  interconvertible  with  the  now  famous  relationship  E  = 
mc2  of  Einstein).  As  expansion  continued,  the  radiant  energy  was  used  to  do  the 
work  of  expansion,  and  matter  became  more  prominent.  At  250  million  years 
the  mass  density  of  matter  and  radiation  became  equal.  Prior  to  that  time  matter 
could  be  thought  of  as  being  "dissolved"  in  thermal  radiation  like  salt  in  water. 

At  this  time  matter  and  gravitation  became  predominant,  and  the  dif- 
ferentiation of  the  previously  homogeneous  system  began.  Gas  balls  were  formed 


ORIGIN    OF    EARTH    AND    UNIVERSE  •   55 

of  the  mass  of  a  galaxy  (about  40,000  light  years  radius  and  200  million  times 
the  mass  of  the  sun).  These  dark  gas  clouds  next  differentiated  or  condensed 
into  stellar  gas  balls  that  contracted  rapidly.  The  compression  raised  the  tempera- 
ture to  20,000,000  degrees,  the  threshold  of  nuclear  reactions,  and  the  stars 
began  to  shine.  As  most  of  the  material  fell  toward  the  center  of  a  star,  the 
planets  were  formed  from  what  was  left  behind.  Colliding  dust  particles  formed 
larger  chunks  of  matter  that  swept  through  space,  growing  larger  all  the  time. 
The  process  of  star  condensation  and  planet  formation  must  have  taken  a  few 
hundred  million  years.  Since  the  moon  is  gradually  moving  further  away  from 
the  earth,  it  appears  that  several  billion  years  ago  earth  and  moon  formed  a 
single  mass,  from  which  the  moon  has  broken  away.  This  conception  of  the 
universe  extends  the  theory  of  evolution  to  the  universe  itself. 

The  steady-state  theory,  on  the  other  hand,  suggests  that  the  universe  is 
infinite  in  both  space  and  time,  that  the  density  of  its  matter  remains  constant, 
and  that  new  matter  is  constantly  being  created  throughout  space  at  a  rate  just 
compensating  for  the  thinning  of  matter  by  expansion,  with  new  galaxies  con- 
stantly being  formed. 

The  major  difficulty  with  the  theory  of  an  initial  30-minute  "cooking 
period"  is  that  there  are  no  stable  atoms  of  mass  5  or  mass  8,  and  therefore  the 
build-up  of  the  heavier  elements  by  neutron  capture  could  not  get  past  helium  4. 
This  shortcoming  in  the  theory  has  led  Gamow,  one  of  its  proponents,  to  agree 
recently  that  the  bulk  of  the  heavy  elements  may  have  been  formed  later  in  the 
hot  core  of  stars. 

Two  recent  tentative  advances  have  lent  still  further  support  to  the 
concept  of  an  evolving  universe.  The  steady-state  hypothesis  postulates  that  the 
density  of  matter  in  space  remains  contant.  The  density  of  radio  stars,  however, 
increases  with  distance.  Since  most  radio  stars  are  apparently  due  to  collisions 
between  galaxies,  this  latter  finding  indicates  that  galactic  crack-ups  were  more 
common  billions  of  years  ago  when  these  signals  started  on  their  way  than  they 
are  today.  Since  the  evolutionary  theory  postulates  a  denser  universe  then,  with 
collisions  between  galaxies  therefore  more  probable,  this  discovery,  if  confirmed, 
gives  strong  support  to  the  theory. 

In  studying  the  red  shift,  distance  is  measured  in  light  years  rather  than 
miles.  The  speed  of  light  is  186,000  miles  per  second,  yet  some  galaxies  are 
millions  of  light  years  away.  In  viewing  these  far-distant  galaxies,  we  are  looking 
not  only  over  great  distances  but  also  backward  in  time.  A  study  of  clusters  of 
galaxies  about  a  billion  light  years  away  has  shown  that  a  billion  years  ago  the 
universe  was  expanding  faster  than  it  is  today.  If  the  rate  of  expansion  is  slow- 
ing down,  then  we  must  live  in  an  evolving  rather  than  a  steady-state  universe. 
Furthermore,  the  slowing  down  suggests  that  eventually  expansion  will  stop  and 
contraction  will  begin,  ultimately  reaching  the  superdense  condition  that  existed 
some  5  or  more  billion  years  ago.  The  concept  of  a  pulsating  universe  is  thus 


56  •  THE    EVIDENCE    FOR   EVOLUTION 

further  strengthened.  As  for  the  question  of  the  structure  of  the  universe  prior 
to  the  colossal  explosion  that  started  it,  it  seems  likely  to  remain  inscrutable,  for 
whatever  previous  structure  existed  was  lost  in  the  dense  mass  of  energy,  elec- 
trons, protons,  and  neutrons  that  gave  rise  to  our  present  universe. 


SUMMARY  < 


Our  knowledge  of  the  origin  of  the  earth  and  of  the 
universe  is  neither  as  specific  nor  as  detailed  as  our  knowledge  of 
the  evolution  of  plants  and  animals  derived  from  the  study  of 
fossils.  Nevertheless,  progress  in  the  fields  of  physics,  chemistry, 
and  astronomy  has  made  it  possible  to  attack  this  question  on  a 
rational,  scientific  basis.  The  results  of  these  studies  indicate  that 
the  earth  was  formed  several  billion  years  ago  and  that  the  age 
of  the  universe  as  we  know  it  is  approximately  5  to  10  billion 
years.  Although  the  alternative  hypothesis  of  a  steady-state  uni- 
verse has  been  advanced,  there  is  considerable  evidence  to  indicate 
that  the  universe  itself  is  an  evolving  system,  changing  through 
time. 

SUGGESTED  READING 

Bondi,  H.,  1952.  Cosmology.  New  York:  Cambridge  University  Press. 
Brown,  H.,  1957.  "The  age  of  the  solar  system,"  Sci.  Amer.,  196(4)  :80-95. 
Gamow,  G.,  1951.  "The  origin  and  evolution  of  the  universe,"  Amer.  Sci.,  39/393- 

406. 

,  1952.  The  creation  of  the  universe.  New  York:  Viking. 

Hoyle,  F.,  1955.  Frontiers  of  astronomy.  New  York:  Harper.  (New  York:  Mentor 

Books,  1957.) 
Pfeiffer,  J.,  1956.  The  changing  universe.  New  York:  Random  House. 
Robertson,  H.  P.  et  al.,  1956.  "The  universe,"  Sci.  Amer.,  195(3)  :72-236. 
Russell,  B.,  1958.  The  ABC  of  relativity.  London:  Allen  and  Unwin.  (New  York: 

Mentor  Books,  1959.) 
Schwarzschild,  M.,  1958.  Structure  and  evolution  of  the  stars.  Princeton:  Princeton 

University  Press. 


CHAPTER 


The  Origin  of  Life 


Since  man  tends  to  seek  final  answers  to  all  major  ques- 
tions, it  is  not  surprising  to  find  some  sort  of  explanation  for  the 
origin  of  the  world,  of  life,  and  of  man  in  practically  every 
human  culture.  These  beliefs  fall  into  the  realm  of  myth  or  super- 
stition in  many  cases  or  they  may  be  a  part  of  the  religion  of  the 
society.  So  intriguing  a  question  as  the  origin  of  life  has  a  number 
of  theories  associated  with  it,  most  of  which  can  be  grouped  into 
a  few  major  categories.  One  category  involves  a  belief  in  the 
creation  of  life  by  a  supernatural  creator,  an  explanation  that  is 
outside  the  realm  of  science  and  therefore  not  open  to  scientific 
study.  Another  category,  however — that  of  spontaneous  generation 
— does  admit  of  such  investigation. 

Spontaneous  Generation 

For  centuries,  the  problem  of  the  origin  of  life  did  not 
loom  large  in  men's  minds,  for  it  was  common  knowledge  that 
life  was  arising  de  novo  all  around  them  all  the  time.  As  if  by 
magic,  worms  appeared  in  their  rain  barrels,  maggots  in  their 
meat,  and  mice  in  their  rag  bags;  hence  the  spontaneous  genera- 
tion of  worms,  maggots,  and  mice,  where  none  had  previously 
existed,  was  a  fact  easily  demonstrated  from  everyday  experience. 
Among  the  Greeks,  Thales,  Anaximander,  Xenophanes,  and  Aris- 
totle all  believed  in  some  form  of  spontaneous  generation.  Even 
such  scientists  as  Harvey,  Newton,  Descartes,  and  Paracelsus  cen- 
turies later  believed  in  it,  and  van  Helmont,  who  did  notable 


57 


58  •  THE    EVIDENCE    FOR   EVOLUTION 

early  work  on  plant  nutrition,  left  a  recipe  for  the  spontaneous  generation  of 
mice — namely,  a  sweaty  shirt  plus  some  wheat  germ. 

Some  of  the  fables  are  so  fantastic  that  it  is  difficult  to  conceive  how 
they  originated.  For  example,  according  to  the  goose  tree  legend  of  the  Middle 
Ages,  geese  were  derived  from  barnacles,  which  in  turn  were  formed  in  the 
fruits  of  trees.  Since  geese  were  thus  obviously  of  vegetable  origin,  for  centuries 


Fig.  6-1.     The  goose  tree  legend. 


they  were  an  acceptable  meat  substitute  during  Lent.  This  belief  was  periodically 
reinforced  by  careful  observations,  often  accompanied  by  imaginative  drawings 
(see  Fig.  6-1),  and  it  persisted  even  to  the  beginning  of  the  seventeenth 
century.  One  possible  explanation  for  the  origin  of  the  legend  is  the  coincidence 
of  the  time  of  attachment  of  the  marine  barnacles  in  the  northern  British  Isles 
with  the  arrival  of  migrating  young  geese  from  the  Arctic.  These  barnacles  attach 
to  a  variety  of  things  in  the  water,  including  fallen  trees  or  branches,  and  this 
fact  may  have  been  the  basis  for  the  strange  juxtaposition  of  beliefs. 

Not  until  the  seventeenth  century  were  the  first  real  doubts  cast  on  the 


THE    ORIGIN    OF    LIFE  •  59 

theory  of  spontaneous  generation.  The  experiments  of  an  Italian,  Francisco  Redi, 
showed  that  meat,  covered  with  a  cloth  so  that  flies  could  not  lay  their  eggs  on  it, 
never  developed  maggots.  The  idea  nevertheless  persisted,  especially  in  relation 
to  microorganisms.  A  century  later,  Spallanzani  sealed  some  broth  in  a  flask, 
boiled  it,  and  showed  that  no  microorganisms  then  developed  and  hence  no 
spoiling  occurred  for  an  indefinite  period.  Needham,  however,  objected  that  the 
broth  and  particularly  the  air  in  the  flask  were  changed  by  the  boiling  so  that 
they  would  not  support  life.  Breaking  the  seal  on  the  flask,  Spallanzani  showed 
that  the  broth  would  still  support  life,  but  he  failed  to  answer  the  criticism  con- 
cerning the  air.  Hence,  belief  in  spontaneous  generation  persisted  not  only 
among  people  generally  but  among  biologists  until  less  than  100  years  ago.  The 
experiments  of  Pasteur  finally  ended  the  argument,  and  the  axiom  became  Omne 
vivum  e  vivo  for  all  beginning  biology  students.  Pasteur's  proof  was  a  simple 
modification  of  Spallanzani's  experiment.  Rather  than  sealing  the  flask,  he  drew 
the  neck  out  into  a  thin  undulating  tube,  open  to  the  air.  After  boiling,  the 
broth  remained  sterile  because  dust  and  bacteria  and  mold  spores  were  trapped 
in  the  neck  of  the  flask  even  though  the  air  molecules  had  free  passage.  After 
Pasteur  had  completed  his  painstaking  series  of  experiments,  no  satisfactory 
explanation  for  the  origin  of  life  remained.  Special  creation  was  not  a  scientific 
explanation,  and  spontaneous  generation  had  been  shown  not  to  occur.  However, 
it  might  be  noted  at  this  point  that  negative  proof  can  never  be  regarded  as  final. 

An  interesting  twist  in  the  theories  was  the  concept  that  nonliving  sub- 
stances came  from  living  things  rather  than  vice  versa,  a  form  of  vitalism  based 
on  the  idea  that  life  itself  is  eternal.  Another  more  or  less  related  theory  involves 
cosmozoa,  living  particles  dispersed  throughout  the  cosmos  that  take  up  their 
abode  and  evolve  whenever  conditions  become  suitable.  Various  methods  of 
their  transmission  through  space  have  been  suggested;  Richter  proposed  floating 
particles,  von  Helmholtz  that  they  arrived  via  meteorites,  and  Arrhenius  that 
they  were  propelled  by  the  pressure  from  light  rays.  There  is,  however,  no  evi- 
dence supporting  the  existence  of  cosmozoa;  indeed,  the  known  effects  of  tem- 
perature, ultraviolet  rays,  and  radiation  on  living  organisms  make  the  theory 
very  improbable.  Even  if  it  were  correct,  the  question  of  the  origin  of  life  is  not 
answered,  but  is  simply  removed  to  some  more  inaccessible  part  of  the  universe 
unless  it  is  assumed,  as  has  been  done,  that  cosmozoa  are  eternal.  The  theory,  in 
sum,  is  far  from  adequate. 

In  recent  years,  a  new  attack  has  been  made  on  the  problem,  and  the 
result  has  been,  interestingly  enough,  a  new  version  of  spontaneous  generation. 
The  theory  proposes  that  life  originated  on  earth  in  the  past  when  conditions 
were  different  from  those  of  the  present,  and  was  preceded  by  a  gradual  chem- 
ical evolution  that  ultimately  gave  rise  to  self-duplicating  molecules.  Pasteur's 
experiments  did  not  eliminate  this  possibility,  for  they  demonstrated  only  that 
life  did  not  originate  spontaneously  under  his  experimental  conditions. 


60  •  THE    EVIDENCE    FOR   EVOLUTION 
The  Composition  of  Living  Things 

In  order  to  discuss  the  conditions  under  which  life  might  have  origi- 
nated in  the  past,  we  must  have  some  idea  of  the  nature  of  living  things.  They 
are  composed  of  water,  inorganic  salts,  and  carbon  compounds — organic  sub- 
stances known  as  carbohydrates,  fats,  proteins,  and  nucleic  acids.  The  nucleic 
acids  in  combination  with  protein  form  the  hereditary  material;  proteins  form 
the  structure  of  the  organism;  and  the  fats  and  the  carbohydrates  such  as  starch, 
glycogen,  and  the  sugars  are  primarily  a  source  of  energy  for  cellular  work. 
These  compounds  are  highly  organized  into  a  smoothly  functioning  whole  in  the 
living  organism.  Thermodynamically,  a  living  animal  is  a  very  improbable 
structure.  The  complex  molecules  are  built  up  from  relatively  few  elements, 
actually  only  20  or  so  out  of  the  95  available  on  the  earth.  Carbohydrates  and 
fats  are  formed  from  carbon  (C),  oxygen  (O),  and  hydrogen  (H)  alone,  and 
these  three  elements  and  the  nitrogen  (N)  essential  to  protein  formation  form 
99  percent  of  living  protoplasm.  Sulfur  and  phosphorus  are  two  other  important 
elements,  found  in  proteins,  for  example.  The  inorganic  salts  are  formed  prima- 
rily from  sodium,  potassium,  calcium,  magnesium,  and  chlorine;  traces  of  iron, 
copper,  manganese,  zinc,  cobalt,  nickel,  iodine,  vanadium,  fluorine,  boron,  alumi- 
num, and  bromine  have  been  found  in  various  species  of  plants  or  animals. 

The  availability  of  the  elements  does  not  determine  their  utilization  in 
living  organisms,  for  some  very  common  elements  in  the  earth's  crust  are  either 
absent  or  present  in  very  low  concentrations  in  organisms.  Hence,  some  sort  of 
selective  process  must  be  involved.  The  unique  feature  about  hydrogen,  oxygen, 
nitrogen,  and  carbon  is  that  they  are  the  smallest  four  atoms  that  can  become 
stable  by  gaining  1,  2,  3,  and  4  electrons  respectively  in  their  outer  shell  of 
electrons.  They  share  electrons  with  other  atoms  to  form  chemical  bonds  that 
lead  to  molecule  formation.  Phosphorus  and  sulfur  are  in  the  same  relative  posi- 
tion in  the  periodic  table  as  nitrogen  and  oxygen,  but  they  are  one  group  higher. 
The  lightest  elements  (C,  H,  O,  N)  are  the  only  ones  that  regularly  share  two 
or  even  three  pairs  of  electrons  with  other  atoms  and  hence  permit  the  building 
up  of  chains  of  atoms.  Silicon  is  chemically  similar  to  carbon  and  much  more 
available  in  the  earth's  crust,  but,  lacking  this  electron-sharing  ability,  is  seldom 
found  in  living  organisms.  The  trace  elements  such  as  the  iron  in  hemoglobin 
or  the  magnesium  in  chlorophyll  are  complex  formers,  holding  together  big 
molecules. 

Water,  which  is  a  major  component  of  organisms,  is  a  unique  sub- 
stance. It  is  the  best  solvent  known,  and  has  a  long  liquid  range — that  is,  a  high 
boiling  point  and  a  low  freezing  point.  It  promotes  the  ionization  of  salts 
through  its  high  dielectric  constant,  and  it  expands  from  4°  C  down  to  0°  C,  its 
freezing  point. 


THE    ORIGIN    OF    LIFE  •  61 

Formation  of  Organic  Compounds 

Our  previous  discussion  of  the  formation  of  the  present  universe  indi- 
cated that  the  elements  were  not  likely  to  be  bound  together  in  large  molecules; 
in  other  words,  organic  compounds  such  as  carbohydrates,  fats,  and  proteins 
were  not  present  on  the  earth  during  its  formative  period.  Life  could  not  have 
originated  on  the  earth  until  the  earth  had  assumed  more  or  less  its  present  form; 
thus,  before  we  can  talk  of  the  origin  of  life,  we  must  discover  what  conditions 
prevailed  on  the  earth  several  billion  years  ago  and  what  means  were  available 
to  cause  the  synthesis  of  the  more  complex  compounds  from  the  very  simple  ones 
that  existed  then.  Unfortunately,  these  questions  are  not  easy  to  answer.  For 
example,  it  is  not  certainly  known  whether  the  earth's  atmosphere  then  contained 
free  oxygen;  prevailing  opinion  is  that  no  free  oxygen  was  present  and  that  the 
atmosphere  was  reducing  in  character.  However,  several  mechanisms  have  now 
been  demonstrated  experimentally  by  which  more  or  less  complex  organic  mole- 
cules can  be  obtained  from  simple  carbon  compounds  such  as  formic  acid  or 
methane  and  nitrogenous  substances  such  as  ammonia  or  nitric  acid  or  nitrates. 
Shown  below  are  some  of  the  structural  formulas  of  compounds  mentioned  in 
the  text. 

H  H  H  O  O 


H— O  N— H 

I 
H 


water        ammonia 

O 

II 

C— OH 

I 
C— OH 


H— C— H 

I 
H 

methane 


o=c=o 


N— OH        H— C— OH 


H 

I 
H— C— H 

I 
C— OH 


O 

oxalic 
acid 


O 

acetic 
acid 


carbon 
dioxide 


O 

II 

C— OH 

H— C— H 

I 
H— C— H 

I 
C— OH 


O 


nitric 
acid 
O 

II 

C— OH 

I 
H— C— OH 

I 
H— C— H 

I 
C— OH 


formic 
acid 


H— N— H 
I 

c=o 

I 

H— N— H 

urea 


Ca 


calcium 
carbide 


H-0=C— H 

acetylene 


O 

succinic 
acid 

H    O 


O 

malic 
acid 


H— C— C— OH 

I 
N— H 

I 
H 

glycine 


H- 


H    H    O 


—OH 


62  •  THE    EVIDENCE    FOR   EVOLUTION 

At  present,  living  things  directly  or  indirectly  get  their  free  energy 
from  sunlight  by  means  of  the  photosynthetic  process  in  green  plants.  Before 
the  evolution  of  photosynthesis,  other  energy  sources  had  to  be  used  because 
simple  molecules  such  as  CH4,  H20,  NH3  and  so  on  do  not  absorb  light  in  the 
visible  spectrum.  Only  after  the  appearance  of  compounds  like  the  porphyrins 
(for  example,  chlorophyll)  or  other  pigments  did  absorption  in  the  visible  spec- 
trum become  possible.  The  energy  sources  that  could  have  made  significant  con- 
tributions to  the  early  synthesis  of  organic  compounds  appear  to  have  been 
primarily  ultraviolet  light  and  electric  discharges  such  as  lightning.  The  possible 
contributions  of  energy  from  cosmic  rays,  radioactivity,  or  volcanoes  seem  to  have 
been  very  slight.  Although  thermal  synthesis  of  organic  compounds  has  been 
suggested,  its  significance  has  been  questioned.  The  surface  of  the  primitive  earth 
is  thought  to  have  been  cool,  as  the  result  of  its  formation  from  the  condensa- 
tion of  a  cold  cloud  of  cosmic  dust,  and  therefore  unfavorable  to  this  type  of 
synthesis. 

A  number  of  experiments  to  demonstrate  possible  methods  for  the 
synthesis  of  organic  compounds  prior  to  the  existence  of  living  organisms  have 
been  performed.  One  type  of  experiment  involved  the  illumination  of  aqueous 
solutions  of  these  simple  compounds  with  ultraviolet  light;  the  result  was  forma- 
tion of  amino  acids  and  heterocyclic  or  ring  compounds.  In  another  experiment, 
water  vapor,  ammonia,  methane,  and  hydrogen,  substances  all  thought  to  have 
been  present  in  the  primitive  reduced  atmosphere,  were  passed  over  an  electric 
spark  to  simulate  the  effects  of  electric  discharges  in  the  upper  atmosphere.  The 
amino  acids,  glycine  and  alanine,  plus  several  others  were  recovered  after  a  week. 
Still  another  method  was  suggested  by  the  Russian  biochemist,  Oparin,  who 
initiated  the  recent  discussions  on  chemical  evolution  with  his  book  The  Origin 
of  Life  published  in  1936.  He  suggested  that  the  earth,  cooling  from  a  hot 
miasma,  had  its  carbon  primarily  in  the  form  of  metallic  carbides,  which,  on 
coming  in  contact  with  water,  formed  the  hydrocarbon,  acetylene.  The  acetylene 
then  could  polymerize  under  the  influence  of  catalysts  to  form  the  longer  carbon 
chain  molecules.  Furthermore,  the  thermal  production  and  conversion  of  amino 
acids  from  malic  acid  and  urea  has  also  been  demonstrated.  Finally,  a  fifth 
method  to  be  tested  experimentally  was  the  effect  of  very  high  energy  radiation 
such  as  that  from  cosmic  rays  or  from  radioactive  minerals.  In  this  manner  solu- 
tions of  carbon  dioxide  and  water  have  been  irradiated  to  form  formic  acid;  the 
formic  acid  has  then  produced  the  2 -carbon  compounds,  oxalic  acid  and  acetic 
acid,  and  even  the  4-carbon  compound,  succinic  acid,  but  all  in  very  low  con- 
centrations. Just  which  conditions  prevailed  and  which  mechanisms  were  impor- 
tant billions  of  years  ago  cannot  yet  be  stated  with  certainty.  The  important 
point  is  that  several  mechanisms  have  been  demonstrated  by  which  organic  com- 
pounds, those  with  carbon-carbon  or  carbon-hydrogen  bonds,  can  be  formed 
without  the  mediation  of  living  organisms. 

Granted,  then,  that  organic  compounds  could  have  been  formed;  the 


THE    ORIGIN    OF    LIFE  •  63 

next  logical  question  concerns  their  stability.  Today,  organic  substances  are 
rapidly  destroyed,  primarily  by  decay  or  oxidation.  Decay  is  due  to  the  activities 
of  living  microorganisms,  but  since  no  life  existed  at  the  time  we  are  discussing, 
the  organic  compounds  were  not  then  subject  to  this  kind  of  decomposition. 
Furthermore,  since  it  is  generally  thought  that  free  oxygen  was  virtually  absent 
from  the  earth's  early  atmosphere,  organic  matter  was  not  subject  to  oxidation 
either,  and  hence  could  accumulate  on  the  earth's  surface.  A  further  point  of 
interest  is  the  belief  that  carbon  dioxide,  like  oxygen,  was  essentially  absent  from 
the  early  atmosphere  though  now  both  are  common  in  the  air.  The  conclusion 
to  be  drawn  is  that  both  oxygen  and  carbon  dioxide  are  present  in  the  atmos- 
phere because  of  the  activities  of  living  organisms;  oxygen  because  of  its  release 
during  photosynthesis  by  plants,  carbon  dioxide  due  to  the  respiration  or  meta- 
bolic activity  of  almost  all  living  things. 

Although  the  early  organic  compounds  were  not  subject  to  decay  or 
oxidation,  they  were  not  entirely  stable.  Just  as  "spontaneous"  formation  of 
organic  matter  was  undoubtedly  possible,  so  was  "spontaneous"  decomposition, 
since  chemical  reactions  are  reversible,  and  some  sort  of  equilibrium  between 
synthesis  and  decomposition  is  achieved.  Furthermore,  because  of  the  energy 
relations  between  the  various  compounds,  the  equilibrium  point  is  usually  far  on 
the  side  of  decomposition.  Thus,  although  amino  acids  have  a  certain  probability 
of  uniting  to  form  polypeptides  or  even  proteins,  the  probability  that  a  protein 
or  polypeptide  will  break  up  into  its  constituent  amino  acids  is  far  greater. 

At  this  point  in  our  chronology  we  have  a  more  or  less  random  assort- 
ment of  simple,  relatively  stable  organic  molecules,  such  as  amino  acids,  in  the 
form  of  a  dilute  aqueous  solution — a  rather  thin  broth — still  a  far  cry  from  even 
the  simplest  of  living  organisms.  Present-day  organisms  can  only  maintain  them- 
selves and  grow  by  a  constant  expenditure  of  energy  drawn  from  their  environ- 
ments. A  living  organism  is,  in  a  sense,  a  chemical  machine,  which,  because  of 
its  organization  and  metabolic  activity,  is  able  to  take  up  materials  and  energy 
from  the  environment  and  incorporate  them  iryorder  to  survive,  grow,  and  repro- 
duce itself.  The  next  question  is  the  crux  of  trie  problem  of  the  origin  of  life: 
How,  from  the  dilute  broth  of  organic  compounds,  did  higher  types  of  organiza- 
tion arise,  persist,  and  ultimately  lead  to  self  duplicating  entities  ?  Unfortunately, 
our  knowledge  here  is  only  a  beginning  toward  complete  understanding.  How- 
ever, various  suggestions  have  been  made  as  to  ways  in  which  large  molecules, 
once  formed,  are  kept  from  breaking  up.  If  the  molecules  are  removed  from 
solution  by  precipitation,  they  no  longer  are  so  apt  to  disintegrate.  Similarly,  by 
becoming  attached  to  other  molecules,  they  are  "trapped"  in  their  more  complex 
form.  In  this  fashion,  molecular  aggregates  of  considerable  complexity  could 
have  been  built  up  in  a  stepwise  fashion.  Furthermore,  the  orderly  propensities 
of  matter — their  tendency  toward  forming  crystals,  for  example — could  also 
have  played  a  role  in  bringing  structure  to  the  random  assortment  of  substances. 
This  order  is  inherent  in  the  molecules.  Muscle  or  cartilage  fibers,  after  being 


64  •  THE    EVIDENCE    FOR   EVOLUTION 

dissolved,  will  return,  on  precipitation,  to  their  original  molecular  patterns. 
Proteins  are  composed  of  long  chains  of  amino  acids  connected  by  peptide 
linkages  (that  is,  a  bond  formed  between  the  carboxyl  group  ( — COOH)  of 
one  amino  acid  and  the  amino  group  ( — NH2)  of  the  next  with  the  elimination 
of  H20).  Since  these  bonds  are  broken  or  hydrolyzed  in  water,  it  has  also  been 
proposed  that  the  long  polypeptide  chains  were  first  formed  by  polymerization 
in,  for  example,  a  dried-up  pool  in  the  absence  of  water  rather  than  in  the 
primordial  "soup." 

Perhaps  the  most  characteristic  trait  of  living  things  is  their  ability  to 
reproduce  their  own  kind.  It  is  at  this  point  that  we  must  begin  to  think  in  terms 
of  chemical  evolution  governed  by  a  selective  process  akin  to  natural  selection. 
Some  chemical  compounds  are  catalysts  for  their  own  formation;  in  a  more  or 
less  random  group  of  molecules  or  aggregates,  an  autocatalytic  compound  will 
have  a  selective  advantage  over  the  others,  for  it  will  tend  to  transform  the 
others  into  itself  or,  in  the  competition  for  substrate,  it  will  win  out  as  each  new 
unit  in  turn  catalyzes  the  formation  of  others  like  itself.  Furthermore,  the  more 
efficient  autocatalysts  will  win  out  in  competition  with  the  less  efficient  types  so 
that  in  time  very  efficient  self-duplicating  systems  will  arise.  If  these  molecular 
aggregates  become  unstable  when  they  exceed  a  certain  size,  they  will  break  up, 
and  the  cycle  of  self-duplication  will  then  start  anew. 

Sources  of  Energy  and  Food 

Finally,  we  should  consider  the  ways  in  which  living  organisms  get  the 
energy  they  need  to  continue  to  exist.  This  energy  must  be  externally  derived 
by  the  organism.  Not  only  must  the  energy  be  obtained,  but  it  must  be  available 
in  such  a  form  that  the  organism  can  make  use  of  it.  Today  living  things  obtain 
their  energy  by  means  of  coupled  reactions  in  which  one  reaction  gives  off 
energy  to  another  that  absorbs  it.  Probably  the  most  important  of  such  coupled 
reactions  in  present  organisms  is  oxidative  phosphorylation,  by  means  of  which 
the  energy  from  burning  (or  oxidizing)  sugar  is  made  available  to  do  cellular 
work  rather  than  being  lost  as  heat.  Instead  of  being  released  in  one  large  burst, 
the  oxidation  is  stepwise,  and  at  each  step  a  little  parcel  of  energy  is  tied  up  as 
chemical  energy  in  a  molecule  known  as  adenosine  triphosphate  (ATP).  The 
formation  of  a  single  peptide  linkage  in  a  protein  requires  a  small  amount  of 
free  energy,  energy  that  can  be  obtained  through  a  coupled  reaction  with  an 
ATP  molecule.  The  energy  exchanges  involving  ATP  are  useful  not  only  in 
protein  synthesis  but  also  in  muscle  contraction  and  in  a  variety  of  other  ways  in 
the  cell.  The  unique  feature  of  the  ATP  molecule  is  that  two  of  its  three  phos- 
phate groups  are  linked  together  by  what  are  known  as  "energy-rich"  or  "high- 
energy"  phosphate  bonds.  The  significant  property  of  these  phosphate  groups  is 
that  in  transfer  to  another  compound  they  carry  with  them  a  certain  amount  of 
free  energy,  and  in  this  way  supply  the  energy  needed  to  do  cellular  work  at  the 


THE    ORIGIN    OF    LIFE  •  65 

time,  in  the  place,  and  in  the  amounts  needed.  The  efficient  energy-coupling 
systems  involving  ATP  and  catalyzed  by  enzymes  undoubtedly  are  the  product  of 
the  evolutionary  process  and  are  probably  derived  from  simpler,  less  efficient 
systems  in  the  past. 

In  addition  to  energy,  the  living  organism  if  it  is  to  live,  grow,  and 
reproduce  requires  food.  The  source  of  food  for  primitive  organisms,  formed 
under  the  conditions  described  previously,  must  have  been  the  other  organic 
molecules  in  the  aqueous  broth.  Since  oxygen  was  absent,  the  only  process  avail- 
able was  fermentation,  by  which  energy  is  obtained  from  the  breakage  and  re- 
arrangement of  organic  compounds  in  the  absence  of  oxygen.  A  typical  fer- 
mentation is  that  of  sugar  by  yeast  to  yield  alcohol,  carbon  dioxide,  and  energy. 

C6H1206    ->      2C02    -f  2C2H5OH  +  energy 

glucose  carbon  ethyl 

dioxide         alcohol 

The  C02  and  alcohol  are  waste  products  in  the  cell  and  must  be  eliminated. 
Fermentation  is  a  destructive  process,  however,  and  the  exhaustion  of  the  avail- 
able organic  compounds  would  have  led  to  a  cessation  of  life. 

The  next  step  must  have  been  the  evolutionary  invention  of  photo- 
synthesis, made  possible  by  the  quantities  of  C02  released  by  fermentation.  Thus 
it  became  possible  for  living  organisms  to  synthesize  their  own  organic  mole- 
cules, using  the  energy  from  the  sun.  The  equation 

6CO2    +  6H20     ■££>     C6H1206  +     602 

light 

carbon        water  glucose        oxygen 

dioxide 

shows  the  synthesis  of  sugar;  nitrogen  was  available  from  inorganic  nitrates  or 
ammonia,  and  therefore  all  of  the  necessary  organic  compounds  could  be  synthe- 
sized. Living  things  now  were  no  longer  dependent  on  the  accumulated  organic 
matter  from  the  nonliving  era,  but  could  synthesize  needed  materials  by  photo- 
synthesis and  obtain  necessary  energy  by  fermentation. 

The  oxygen  production  by  photosynthesis  provided  a  much  more  effi- 
cient source  of  energy,  however.  The  waste  products  of  fermentation — alcohol, 
lactic  acid,  formic  acid,  etc. — are  poisonous,  and  the  energy  yield  is  low.  The 
process  of  respiration,  or  the  combination  with  oxygen,  is  much  more  efficient, 
for  the  energy  produced  is  about  35  times  as  great  for  the  same  amount  of  sugar 
consumed.  All  possible  energy  is  extracted;  thus  a  maximum  amount  of  energy 
is  obtained  from  a  minimum  amount  of  material.  Furthermore,  the  waste  prod- 
ucts, carbon  dioxide  and  water,  are  harmless  and  easily  disposed  of.  The  equation 
for  respiration  is 

C6H1206  +     602       ->       6C02    +  6H20  +  energy 

glucose        oxygen  carbon        water 

dioxide 


66  •  THE    EVIDENCE    FOR   EVOLUTION 

The  processes  of  photosynthesis  and  respiration  have  made  life,  as  we 
know  it  today,  possible.  In  tending  to  pride  ourselves  on  our  progress  and  on 
our  control  over  the  environment,  we  sometimes  overlook  man's  complete  de- 
pendence on  energy  from  the  sun  for  his  very  existence.  Since  fermenting  organ- 
isms have  never  evolved  to  a  very  high  degree  of  organization  and  complexity, 
it  seems  reasonable  to  suppose  that  only  with  the  origin  of  respiration  did  the 
evolution  of  more  complex  organisms,  including  man,  become  possible. 

Therefore,  the  current  hypotheses  of  the  origin  of  life  envision  initially 
the  random  formation  of  more  or  less  complex  organic  compounds  from  the 
simpler  molecules  present  in  what  was  probably  a  reducing  atmosphere.  Auto- 
catalytic  molecules,  having  a  selective  advantage  over  the  other  types,  tended  to 
increase  in  frequency.  At  what  point  one  should  stop  speaking  of  molecules  and 
start  referring  to  living  organisms  is  rather  difficult  to  say.  However,  since  a  self- 
duplicating  system  capable  of  mutation  is  frequently  regarded  as  the  fundamental 
criterion  for  life,  by  this  standard  we  are  already  discussing  living  systems.  The 
original  organisms  were  heterotrophic,  obtaining  their  essential  constituents  from 
the  environment  rather  than  synthesizing  them  from  carbon  dioxide  and  water. 
Evolution  of  additional  enzyme  systems  as  a  result  of  the  selective  process  then 
led  to  autotrophic  organisms  capable  of  carrying  out  increasingly  complex  and 
efficient  syntheses  from  very  simple  precursor  substances.  The  exact  steps  by 
which  cellular  life  as  we  know  it  today  arose  through  the  process  of  chemical 
evolution  cannot  be  stated  with  certainty.  Nevertheless,  some  of  the  basic  ques- 
tions involve  the  origin  of  protein  synthesis,  of  deoxyribonucleic  acid  as  the 
genetic  material,  of  high-energy  organic  phosphates  such  as  ATP,  of  catalytic 
compounds  or  enzymes,  particularly  the  porphyrins,  and  the  origin  of  cell  struc- 
ture. Although  answers  to  these  questions  are  at  present  rather  speculative,  active 
research  in  this  field  is  in  progress,  and  at  a  recent  symposium  on  evolution,  a 
panel  of  experts  was  unanimous  in  agreeing  that  the  synthesis  of  life  was  both 
conceivable  and  possible  in  the  not  too  distant  future. 

Hence,  the  origin  of  life  cannot  be  regarded  as  a  mysterious,  unique 
process  but,  rather,  one  that  was  practically  inevitable  and,  moreover,  will  occur 
whenever  and  wherever  similar  conditions  exist.  Since  billions  of  planets  like  the 
earth  are  scattered  throughout  the  universe,  it  is  conceivable  that  life  exists  in 
many  more  places  than  the  earth.  The  astronomer  Harlow  Shapley  has  estimated 
very  conservatively  that  there  are  approximately  100,000,000  planets  in  the  uni- 
verse capable  of  supporting  life  similar  to  that  on  the  earth.  None  of  the  details 
of  this  account  can  be  taken  too  seriously  or  as  finally  established,  and  to  some 
people  it  may  seem  no  more  than  a  modern  fable  of  the  origin  of  life,  com- 
parable to  those  of  the  ancients  and  with  a  similar  purpose.  Nevertheless,  there 
is  sufficient  evidence  to  consider  it  a  reasonable  hypothesis  worthy  of  further 
study. 


THE    ORIGIN    OF    LIFE  •  67 

►  SUMMARY 


Again,  as  with  the  origin  of  the  universe,  recent  scientific 
advances  have  made  it  possible  to  attempt  to  answer  the  question 
of  the  origin  of  life  on  a  rational  basis  and  even  to  tackle  it  ex- 
perimentally. Present  theories  recognize  that  life  arose  when  the 
physical  conditions  on  the  earth  were  quite  different  from  those  at 
present.  A  long  period  of  chemical  evolution  is  thought  to  have 
preceded  the  origin  of  the  first  self-duplicating  particles  that 
could  be  called  living.  The  earliest  forms  of  life  are  thought  to 
have  been  saprophytic,  deriving  energy  from  the  fermentation  of 
organic  compounds  in  the  environment.  Only  later  did  living 
cells  evolve  the  ability  to  synthesize  complex  molecules  from 
simple  precursors,  a  trend  that  culminated  in  the  evolutionary  in- 
vention of  photosynthesis.  Respiration,  a  far  more  efficient  process 
of  energy  extraction  than  fermentation,  only  became  possible  after 
the  oxygen  in  the  atmosphere  increased  as  a  result  of  photo- 
synthesis. 

SUGGESTED  READING 

Blum,  H.  F.,  1955.  Time's  arrow  and  evolution,  2d  ed.  Princeton:  Princeton  Univer- 
sity Press. 

Calvin,  M.,  1956.  "Chemical  evolution  and  the  origin  of  life,"  Amer.  Set.,  44:248- 
263. 

,   1959.  "Evolution  of  enzymes  and  the  photosynthetic  apparatus,"  Science, 

230:1170-1174. 

,  1959.  "Round  trip  from  space,"  Evolution,  23:362-377. 

Fox,  S.  W.,  1956.  "Evolution  of  protein  molecules  and  thermal  synthesis  of  bio- 
chemical substances,"  Amer.  Sci.,  44:347-359. 

Gaffron,  H.,  I960.  "The  origin  of  life,"  Evolution  after  Darwin,  Vol.  I,  The  Evolu- 
tion of  life.  S.  Tax,  ed.  Chicago:  University  of  Chicago  Press. 

Miller,  S.  L.,  1953.  "A  production  of  amino  acids  under  possible  primitive  earth 
conditions,"  Science,  2  27:528-529. 

,  and  H.  C.  Urey,  1959.  "Organic  compound  synthesis  on  the  primitive  earth," 

Science,  230:245-251. 

Oparin,  A.  I.,  1957.  The  origin  of  life  on  the  earth,  3d  ed.  New  York:  Academic 
Press. 

,  et  al.,  eds.,  1959.  The  origin  of  life  on  the  earth.  Pergamon  Press.  Reports 

of  the  Moscow  Symposium  on  the  origin  of  life.  August  1957. 

Pringle,  J.  W.  S.,  1953.  "The  origin  of  life,"  Symposium  Soc.  Exp.  Biol.,  7  (Evolu- 
tion): 1-21.  New  York:  Academic  Press. 

Wald,  G.,  1954.  "The  origin  of  life,"  Sci.  Amer.,  292(2)  :44-53. 


CHAPTER 


7 

Geographical  Distribution 


The  physical  evidence  for  evolution  consists  of  living 
organisms  and  the  remains  of  organisms  that  have  lived  in  the 
past.  Although  the  fossil  record  presents  concrete  evidence  that 
species  differing  from  all  living  species  lived  long  ago,  it  is  often 
sketchy  or  incomplete  on  critical  points.  If  the  record  were  com- 
plete, we  would  have  before  us  the  complete  phylogeny  of  all 
living  things  and  there  would  be  no  need  to  seek  further  infor- 
mation by  more  indirect  methods.  However,  because  of  the 
paucity  of  the  fossil  record,  it  has  been  necessary  to  turn  to  living 
organisms  to  plot  more  fully  the  course  of  past  evolution.  The 
study  of  the  present  geographical  distribution  of  animals  and 
plants  has  lent  considerable  support  to  the  theory  of  evolution. 

In  our  discussion  of  adaptation  we  noted  that  organisms 
are  adapted  to  their  environments.  It  is  now  necessary  to  analyze 
this  situation  still  further.  Within  a  given  geographical  area,  the 
environment  is  not  uniform;  in  other  words,  a  great  variety  of 
different  types  of  habitat  exist.  In  the  state  of  Minnesota,  for  ex- 
ample, three  major  types  of  terrestrial  habitat  can  be  recognized: 
the  deciduous  forest  in  the  southeast,  the  coniferous  forest  to  the 
north,  and  the  prairie  in  the  west  and  southwest.  If  the  variety  of 
fresh-water  habitats  to  be  found  in  the  thousands  of  lakes,  and  in 
the  streams,  swamps,  bogs,  and  rivers  is  included,  the  range  of 
possible  habitats  becomes  even  wider.  Yet  each  species  has  its  own 
ecological  niche,  its  own  unique  requirements  of  the  environment; 
where  these  are  not  met,  that  species  is  not  to  be  found.  To  use 
a  painfully  obvious  example,  the  fish  in  Minnesota  are  confined 

68 


GEOGRAPHICAL    DISTRIBUTION  •  69 

to  the  water.  Much  more  subtle  differences  than  that  between  fresh  water  and  dry 
land  may  determine  whether  a  species  will  be  found  in  a  particular  spot;  thus, 
within  a  given  area  such  as  Minnesota,  the  ecological  conditions  may  vary  widely, 
and  the  species  present  will  vary  also  in  accordance  with  the  changes  in  ecolog- 
ical factors.  Though  no  physical  barrier  exists,  the  animals  and  plants  to  be  found 
in  the  deciduous  forest  areas  of  southeastern  Minnesota  are  distinctly  different 
from  the  animals  and  plants  to  be  found  in  the  coniferous  forests  to  the  north, 
and  surprisingly  few  species  are  common  to  both  areas. 

But,  and  this  is  a  very  important  "but,"  there  is  another  aspect  to  dis- 
tribution, which  can  be  most  readily  outlined  by  quoting  from  Darwin. 

Neither  the  similarity  nor  the  dissimilarity  of  the  inhabitants  of  various 
regions  can  be  wholly  accounted  for  by  climatal  and  other  physical  conditions  .... 
There  is  hardly  a  climate  or  condition  in  the  Old  World  which  cannot  be  paralleled 
in  the  New — at  least  as  closely  as  the  same  species  generally  require  ....  Not- 
withstanding this  general  parallelism  in  the  conditions  of  the  Old  and  New  Worlds, 
how  widely  different  are  their  living  productions. 

For  example,  the  climates  of  parts  of  Australia,  South  Africa,  and 
western  South  America  are  very  much  the  same,  but  the  fauna  and  flora  in  each 
region  are  strikingly  different.  In  South  America,  on  the  other  hand,  the  species 
south  of  35°  latitude  and  those  north  of  25°  latitude  are  clearly  quite  similar, 
although  they  live  under  markedly  different  climatic  conditions. 


Biogeographical  Realms 

Because  species  living  in  the  same  region  tend  to  resemble  each  other 
despite  considerable  differences  in  climate  and  habitat,  it  has  been  possible  to 
delimit  biogeographical  realms,  within  which  the  existing  groups  of  animals  and 
plants  show  many  similarities.  These  realms,  shown  in  Fig.  7-1,  are  the 

1.  Nearctic — North  America  down  into  the  Mexican  plateau  in  central  Mexico. 

2.  Palearctic — Asia  north  of  the  Himalayas,  Europe,  and  Africa  north  of  the 

Sahara  Desert.  Since  the  species  of  the  Nearctic  and  Palearctic  regions 
are  much  alike  in  many  respects,  these  two  regions  are  sometimes 
grouped  together  as  the  Holarctic. 

3.  Neotropical — Central  and  South  America. 

4.  Ethiopian — Africa  south  of  the  Sahara. 

5.  Oriental — Asia  south  of  the  Himalayas. 

6.  Australian. 

Though  the  absence  of  a  species  because  of  an  unsuitable  environment 
is  easy  to  appreciate,  its  absence  when  the  environment  is  favorable  poses  other 
questions.  There  is  little  doubt  that  many  species  can  survive  and  even  thrive  in 


70  •  THE    EVIDENCE    FOR   EVOLUTION 

regions  other  than  the  one  in  which  they  normally  occur.  The  rapid  increase  and 
spread  across  the  United  States  of  the  English  sparrow  and  the  starling  intro- 
duced from  Europe  within  the  past  century  is  a  case  in  point.  Further  examples 
are  the  depredations  of  the  Japanese  beetle  and  the  gypsy  moth,  two  other  species 
recently  introduced  into  the  United  States.  Many  of  our  common  roadside  weeds 
and  flowers  also  had  their  origin  in  Europe,  but  were  brought  here  with  seeds  or 
escaped  from  gardens.  The  phenomenal  increase  in  the  number  of  rabbits  in 
Australia,  where  they  have  become  a  serious  pest  in  the  absence  of  the  predators 
found  in  their  usual  range,  is  striking  evidence  that  ecological  factors  alone  do 
not  determine  the  distribution  and  numbers  of  animals. 


.^^^lEARCTIC  n 


ETHIOPIAN     /ORIENTAL' 


AUSTRALIAN/ 


Fig.  7-1.     The  biogeographical  realms. 

Table  7-1  shows  the  distribution  of  some  significant  groups  of  mam- 
mals; a  few  comments  may  help  emphasize  some  of  its  important  aspects.  The 
similarities  between  the  Nearctic  and  Palearctic  are  quite  obvious.  The  single 
metatherian  or  marsupial  in  the  Nearctic  is  the  opossum,  and  the  edentate  is  the 
armadillo,  both  of  which  appear  to  have  spread  north  from  South  America.  The 
few  primates  of  the  Palearctic  are  found  on  the  fringes  of  the  Ethiopian  and 
Oriental  realms.  Although  no  members  of  the  camel  group  now  exist  in  the 
Nearctic,  large  numbers  of  fossils  indicate  their  presence  in  the  past.  Not  only 
the  same  major  groups  but  closely  similar  species  within  these  groups  are  to  be 
found  in  the  Nearctic  and  Palearctic. 

The  Neotropical  realm  is  a  curious  mixture  of  "modern"  and  "primi- 
tive" mammals.  The  edentates  are  also  characteristic  and  quite  numerous. 


GEOGRAPHICAL   DISTRIBUTION   •  71 

The  Ethiopian  or  African  region  has  the  richest  mammalian  fauna  but 
lacks  completely  the  monotremes  and  marsupials.  The  hoofed  mammals  or 
ungulates  are  a  large  and  important  group  with  many  representatives  of  both 
the  Perissodactyla  (odd-toed)  and  Artiodactyla  (even-toed)  orders.  There  are 
many  rodents,  carnivores,  insectivores,  and  primates. 

In  the  Oriental  region,  similarities  to  both  the  Ethiopian  and  Palearctic 
realms  can  be  seen.  For  example,  elephants,  rhinoceroses,  and  antelope  are  com- 
mon to  the  Ethiopian  and  Oriental;  deer  (Cervidae),  and  sheep  and  goats  to  the 
Palearctic  and  Oriental  (and  Nearctic). 

TABLE    7-  1 
Distribution  of  Certain  Mammalian  Groups 


Bio  geographical  Realm 

Group 

Neotropical 

Nearctic 

Palearctic 

Ethiopian 

Oriental 

Australian 

Monotremes 

_ 

_ 

_ 

— 

— 

2 

Marsupials 

+ 

1 

— 

— 

2 

+ 

Edentates 

+ 

1 

— 

— 

— 

— 

Bats 

+ 

+ 

+ 

+ 

4- 

+ 

Ungulates 

few 

+ 

+ 

+ 

+ 

— 

(Artiodactyls  & 

Perissodactyls) 

Rodents 

© 

+ 

+ 

+ 

+ 

© 

Carnivores 

+ 

+ 

+ 

+ 

+ 

— 

Primates  a 

+ 

— 

few 

+ 

+ 

— 

Insectivores 

few 

+ 

+ 

+ 

+ 

— 

Lagomorphs 

+ 

+ 

+ 

+ 

+ 

(intro- 
duced) 

Tapirs 

+ 

— 

— 

— 

+ 

— 

Camels 

+ 

— 

+ 

— 

— 

— 

+ -representatives  of  group  are  present. 

representatives  of  group  are  absent. 

1,  2 -only  1  or  2  species  of  group  are  present. 

©-group  is  well  represented  but  species  differ  markedly  from  those  in  other  parts  of  the  world. 

a- exclusive  of  man. 

In  Australia,  very  few  groups  are  represented;  the  marsupials  pre- 
dominate, and  only  the  rodents  and  bats  are  well  represented  among  the  Eutheria. 
The  only  living  egg-laying  mammals  or  monotremes  are  found  there. 

From  this  brief  sketch  of  mammalian  distribution,  it  is  clear  that  the 
different  regions  of  the  world  have  their  own  distinctive  faunas,  though  adja- 
cent regions  tend  to  show  more  similarities  than  more  remote  areas.  It  should 
also  be  noted  that  the  Chiroptera,  the  bats,  are  the  only  order  rather  uniformly 
distributed  throughout  the  world.  The  widely  distributed  rodents  have  endemic 
groups  (that  is,  peculiar  to  a  particular  locality) ,  especially  in  Australia  and  the 
Neotropical  region.  Similar  findings  have  emerged  from  the  study  of  other 
animal  and  plant  groups.  The  above  facts  suggest  that  in  addition  to  the  ecolog- 


72  •  THE    EVIDENCE    FOR   EVOLUTION 

ical  factors  that  set  limits  on  distribution,  the  other  major  limiting  factor  on 
distribution  is  what  we  may  term  the  historical.  A  species  or  group  will  only  be 
present  in  a  given  region  if,  at  some  time  in  the  past,  it  was  able  to  reach  that 
region.  For  most  species,  oceans  or  deserts  or  mountain  ranges  have  been  bar- 
riers to  the  further  expansion  of  their  ranges.  The  bats,  however,  with  their  great 
mobility,  have  spread  easily  throughout  the  world,  even  to  the  most  remote 
oceanic  islands.  This  explanation  raises  almost  as  many  problems  as  it  solves,  for 
the  implication  is  that  each  species  has  had  a  single  center  of  origin.  The  ques- 
tions that  arise  in  connection  with  any  species  are,  then,  where  was  its  center  of 
origin  and  when  did  it  originate. 

Present  distribution  is  intelligible  only  on  the  assumptions  that  each 
species  has  originated  only  once,  that  species  have  had  their  origins  in  practically 
all  habitable  parts  of  the  earth,  and  that  they  have  originated  throughout  the 
geological  history  of  the  earth.  Each  species  tends  to  expand  like  a  gas  from  its 
center  of  origin,  the  pressure  being  due  to  its  high  reproductive  capacity;  migra- 
tion will  then  fill  all  available  areas  until  further  expansion  is  blocked  by 
physical  barriers  or  by  unfavorable  environmental  conditions.  New  species  can 
evolve  only  after  a  population  of  an  existing  species  has  become  to  some  degree 
physically  isolated  from  the  parental  species.  Hence  related  species  or  groups 
will  tend  to  be  found  in  adjacent  areas.  We  will  now  consider  some  specific  ex- 
amples of  geographical  distribution  and  see  how  they  are  explained  in  terms  of 
the  theory  of  evolution  plus  a  knowledge  of  the  geological  history  of  the  earth. 

Primitive  and  Modern  Mammals  in  the  Neotropical 

The  mixture  of  "primitive"  and  "modern"  mammals  in  the  Neotropical 
region  has  already  been  mentioned.  The  "primitive"  group  includes  anteaters, 
sloths,  armadillos,  many  marsupials,  primitive  primates  (platyrrhine  monkeys 
and  marmosets),  and  a  unique  group  of  rodents.  All  of  these  are  peculiar  to 
South  America.  The  "modern"  group  is  very  similar  to  the  fauna  of  North 
America  though  for  the  most  part  the  species  are  different.  Included  are  deer, 
various  cats,  wolves,  otters,  many  rodents,  guanacos,  and  llamas. 

With  the  assumption  of  evolution,  the  explanation  is  relatively  simple. 
Marine  fossils  similar  to  those  of  the  Miocene  elsewhere  are  found  on  land  in 
Panama;  thus  Panama,  the  link  between  North  and  South  America,  must  have 
been  submerged  during  the  mid-Tertiary.  The  "primitive"  group  of  mammals 
reached  South  America  in  the  late  Cretaceous  and  Paleocene  from  North  America 
and  then  evolved  in  isolation  during  the  period  of  submergence.  Re-emergence 
of  the  land  gave  rise  first  to  island  chains  and  then  Panama  rose  again  above  the 
surface  of  the  sea  during  the  Pleistocene.  The  "modern"  mammals  invaded 
South  America  via  this  new  land  bridge.  Many  of  the  "primitive"  forms  in 
South  America  could  not  compete  with  the  more  efficient  new  immigrants  and 


GEOGRAPHICAL    DISTRIBUTION  •  73 

became  extinct.  Their  history  is  known  from  the  extensive  fossil  record.  Only  a 
few  species  of  the  South  American  fauna  were  adaptable  enough  to  spread  their 
ranges  into  North  America,  among  them  the  armadillo,  the  opossum,  and  the 
porcupine. 

Nearctic  and  Palearctic 

The  similarities  in  the  biota  of  North  America  and  Eurasia  have  already 
been  mentioned  as  warranting  the  inclusion  of  both  areas  in  one  biogeographical 
realm,  the  Holarctic.  Though  these  two  land  masses  are  now  isolated,  the  evi- 
dence is  clear  that  in  the  late  Tertiary,  a  land  bridge  in  the  Bering  Sea  region 
was  repeatedly  formed  and  broken.  The  fossil  record  indicates  that  the  camels 
originated  in  North  America  and  flourished  here,  evolving  into  a  variety  of 
species,  some  of  which  migrated  to  South  America  or  to  Asia.  At  present  the 
group  is  entirely  extinct  in  North  America,  but  the  curiously  disjunct  distribution 
of  the  group  is  intelligible  when  the  fossil  record  and  geological  events  are 
known.  However,  migration  more  frequently  was  from  Asia  to  North  America; 
the  bison,  mammoths,  bears,  cats,  and  deer,  for  example,  originated  in  the 
Eurasian  land  mass  and  spread  to  North  America.  It  should  be  realized  that 
during  this  period  climatic  conditions  underwent  changes  as  well.  Early  in  the 
Cenozoic,  North  America  was  relatively  flat,  and  the  Bering  land  bridge  formed 
a  broad  connection  between  the  two  continents.  Fossils  deposited  at  that  time 
indicate  that  the  climate  was  much  milder  than  at  present,  for  the  fossil  record 
shows  that  alligators,  sassafras  trees,  and  magnolias  were  more  or  less  continu- 
ously distributed  from  southeastern  United  States  to  eastern  China,  from  the 
banks  of  the  Yangtse  to  the  banks  of  the  Suwanee.  In  the  late  Cenozoic,  the 
Rockies  rose,  western  North  America  became  colder  and  drier,  and  these  species 
were  eliminated  from  much  of  their  former  range.  Next  came  the  glaciers,  which 
wiped  out  practically  everything  in  their  paths.  In  North  America  their  extreme 
southern  limits  were,  roughly,  the  Ohio  River  and  the  Missouri  River  (see 
Fig.  7-2).  During  this  invasion  by  the  ice,  southeastern  United  States  and 
eastern  China  were  only  slightly  affected,  and  in  these  two  areas  the  alligators, 
the  sassafras,  and  the  magnolias  survived.  In  the  million  years  since  these  popu- 
lations became  isolated  from  each  other  they  have  evolved  to  the  extent  that  they 
are  now  recognized  as  distinct  species  of  the  same  genus;  there  are  other  species 
(for  example,  skunk  cabbage)  that  are  apparently  the  same  in  both  areas. 

Relict  Alpine  Populations 

In  the  Northern  Hemisphere  it  is  often  observed  that  species  at  the 
higher  altitudes  in  mountainous  areas  are  similar  to  those  at  lower  altitudes 
farther  north  rather  than  to  the  species  living  at  the  foot  of  the  mountains.  For 


74  •  THE    EVIDENCE    FOR    EVOLUTION 

North  Pole! 


□  ' 


Ice  pack 

%  Not  covered 
by  ice 


Fig.  7-2.     The  approximate  extent  of  the  glaciers  in  North  America  during  the 

Pleistocene. 

example,  some  of  the  species  in  the  Great  Smoky  Mountains  of  Tennessee  are 
only  found  again  hundreds  of  miles  to  the  north  in  Canada,  and  some  plants  on 
Mt.  Washington  in  the  White  Mountains  of  New  Hampshire  are  isolated  popu- 
lations of  species  found  in  Labrador.  It  seems  probable  that  species  adapted  to 
arctic  or  subarctic  conditions  retreated  to  the  south  as  the  glaciers  advanced,  and 
were  forced  into  more  southern  areas  of  North  America.  As  the  glaciers  re- 
treated, the  species  migrated  north  and  also  up  the  mountainsides,  continuing  to 
survive  in  areas  to  which  they  were  adapted.  In  this  way  relict  populations  were 
left  behind  on  the  mountains. 


Primitive  Southern  Fauna 

Not  only  South  America  but  the  other  southern  land  masses,  Africa 
and  Australia,  have  "primitive"  fauna,  each,  however,  quite  unique.  Australia's 
mammals  are  primarily  marsupials,  and  among  the  insects  are  found  primitive 
bees,  termites,  and  butterflies.  In  Africa,  "primitive"  primates  such  as  the  lemurs, 
and  other  species  such  as  the  aardwolf  and  the  chevrotain  still  exist.  Each  of 
the  three  areas  has  a  genus  of  the  lungfish.  This  concentration  of  primitive 


GEOGRAPHICAL    DISTRIBUTION  •  75 

species  in  the  Southern  Hemisphere  has  led  some  investigators  to  believe  that 
these  land  masses  were  at  one  time  united  but  later  split  apart  and  gradually 
drifted  northward  to  their  present  positions.  This  interesting  theory  of  "Conti- 
nental Drift"  postulates  that  at  one  time  there  were  two  major  land  masses — 
Gondwana,  centering  on  the  South  Pole,  and  Laurasia  in  the  vicinity  of  the 
equator.  These  masses  drifted  gradually  northward,  Laurasia  splitting  into  North 
America  and  Eurasia,  and  Gondwana  splitting  up  to  form  Africa,  South  America, 
Antarctica,  and  the  Arabian  and  Indian  peninsulas.  The  drifting  was  very  slow 
and  not  completed  until  the  Tertiary.  Although  the  fossils  of  tropical  species  in 
Alaska  and  the  lungfish  genera  and  other  similarities  between  Australia,  Africa, 
and  South  America  could  be  explained  on  this  basis,  the  geological  evidence  for 
the  split  is  not  impressive  and  the  theory  poses  about  as  many  biogeographical 
problems  as  it  solves. 

Some  form  of  Matthew's  theory  of  climate  and  evolution  seems  a  more 
reasonable  explanation  for  the  geographical  distribution  of  living  and  fossil 
species.  Matthew  suggested  that  the  continents  and  ocean  basins  have  occupied 
relatively  permanent  positions  at  least  since  the  Mesozoic,  but  that  the  climate  of 
the  earth  has  fluctuated  between  warm,  moist  periods  and  cold,  dry  periods. 
During  the  warm  phases,  the  seas  have  covered  the  continental  lowlands,  and 
tropical  and  subtropical  species  have  expanded  their  ranges  far  to  the  north. 
During  the  cold  phases,  the  continents  were  elevated,  glaciers  expanded  south- 
ward, and  only  the  tropics  remained  mild.  The  land  masses  were  primarily  north 
of  the  equator,  and  the  southern  continents  remained  more  or  less  isolated  and 
warm  even  during  the  cold  periods.  In  the  glacial  periods,  species  had  to  adapt 
to  the  changing  conditions,  or  migrate,  or  perish.  Major  new  evolutionary  types 
seem  to  have  appeared  on  the  major  land  masses,  the  southern  continents  serv- 
ing as  refuges.  This  theory  explains  geographical  distribution,  then,  by  means  of 
climatic  changes  and  known  land  bridges,  with  no  major  shifts  in  the  position  of 
the  continents  or  the  oceans.  The  most  probable  explanation  of  the  fossil  record 
appears  to  be  that  the  earliest  mammals — monotremes  and  marsupials — origi- 
nated in  Eurasia  or  North  America  and  were  able  to  migrate  into  all  the  major 
land  areas.  When  the  placental  mammals  arose,  also  in  the  Northern  Hemis- 
phere, they  replaced  the  marsupials  in  the  Holarctic;  the  former  connection  to 
Australia  was  completely  broken,  however,  and  Africa  and  South  America  were 
partially  isolated  by  barriers  of  desert  or  water,  and  the  more  "primitive"  forms 
there  were  at  least  partly  protected  from  competition  with  the  more  "modern" 
and  efficient  mammals  that  continued  to  evolve  to  the  north. 

Continental  and  Oceanic  Islands 

Two  distinct  types  of  islands,  continental  and  oceanic,  can  be  identified. 

The  continental  islands  are  generally  separated  from  a  continent  by  a  shallow 


76  •  THE   EVIDENCE    FOR   EVOLUTION 

sea.  The  rock  formations  on  both  the  land  mass  and  the  island  are  similar,  with 
the  islands  basically  formed  from  stratified  rock.  The  continental  islands  are 
separated  from  the  mainland  if  the  sea  level  rises  or  the  land  sinks.  Typical  of 
the  continental  type  are  the  British  Isles,  Borneo,  Sumatra,  and  Java.  Oceanic 
islands  are  usually  volcanic  in  origin,  hence  formed  of  igneous  rock,  and  are  sepa- 
rated from  the  major  land  masses  by  deep  water.  The  Hawaiian  Islands  and  the 
Galapagos  Islands  are  examples  of  oceanic  islands.  Not  only  do  continental  and 
oceanic  islands  differ  in  their  mode  of  origin,  but  they  have  quite  different  types 
of  fauna. 

Each  of  the  oceanic  islands  or  island  groups  has  its  own  distinctive 
fauna,  different  from  the  faunas  in  all  other  parts  of  the  world.  Compared  with 
the  continents,  the  oceanic  islands  have  depauperate  faunas.  There  are  seldom 
any  mammals  except  bats,  though  rodents,  possibly  introduced  by  man,  are  some- 
times present.  The  only  fresh-water  fishes  are  those  capable  of  adapting  to  life 
in  salt  water.  Such  small  animals  as  snails,  lizards,  insects,  and  land  birds  are 
found.  The  fauna  of  continental  islands  is  clearly  derived  from  the  nearby  conti- 
nent; though  the  species  may  sometimes  be  different,  the  similarities  are  quite 
striking.  There  is  a  distinct  relationship  between  distance  and  the  similarity  of 
the  species  on  island  and  mainland.  The  British  Isles  have  essentially  the  same 
species  as  the  European  mainland;  Ireland,  however,  lacks  some  elements  found 
on  the  continent.  Though  St.  Patrick  has  long  received  credit  for  the  absence  of 
snakes  there,  their  inability  to  cross  an  ocean  barrier  in  postglacial  times  is  a 
more  reasonable,  though  less  romantic,  explanation.  Where  the  distance  is 
greater  or  the  connection  to  the  continent  less  recent,  as  in  Sumatra,  Java,  or 
Borneo,  different  species  have  had  a  chance  to  evolve,  but  they  are  similar  to  the 
mainland  species  that  originally  populated  the  island  and  from  which  they  are 
descended.  On  Sumatra,  for  example,  a  small  edition — a  different  species — of 
the  rhinoceros  found  on  the  mainland  has  evolved.  For  some  reason,  island 
species  are  frequently  smaller  than  their  close  relatives  on  the  mainland,  but  the 
adaptive  significance  of  this  tendency  requires  further  study. 

After  a  volcanic  eruption  the  oceanic  islands  must  have  formed  a 
barren  mass  of  rock  in  the  vast  distances  of  the  sea.  The  explosion  of  Krakatoa 
in  1883  has  provided  an  actual  example  of  such  an  event  for  study.  Once 
formed,  the  island  will  become  inhabited  only  by  those  species  capable  in  one 
way  or  another  of  traversing  the  formidable  barrier  of  ocean  and  sheer  distance 
that  confronts  the  terrestrial  species.  Chance  thus  plays  a  large  role  in  determin- 
ing which  species  happen  to  bridge  the  gap.  Some  groups,  however,  are  much 
more  capable  of  wide  dispersal  than  others;  for  example,  the  probability  is  great 
that  such  groups  as  birds  and  bats  will  be  present,  but  it  is  practically  zero  for  ele- 
phants. Among  the  birds,  chance  again  may  play  a  major  role  in  determining 
which  species  reach  the  island.  The  Hawaiian  honey  creepers  and  Darwin's 
finches  on  the  Galapagos  Islands  are  instances  of  arrays  of  species  that  have 


GEOGRAPHICAL   DISTRIBUTION  •  77 

evolved  on  the  islands  from  original  immigrant  groups,  perhaps  even  a  single 
flock  wandering  or  blown  far  from  its  usual  haunts. 

Thus,  the  present  distribution  of  species  is  most  intelligible  if  inter- 
preted in  terms  of  the  ecological  conditions,  the  historical  factors  that  have 
limited  their  expansion,  and  the  theory  of  evolution.  Within  this  framework,  the 
peculiarities  of  island  distribution,  alpine  distribution,  regional  similarities,  and 
the  many  other  facets  of  biogeographical  distribution  can  be  fitted.  No  other 
system  has  a  logical,  rational  explanation  for  so  many  of  the  facts. 


►  SUMMARY 


Plants  and  animals  are  not  uniformly  distributed  over  all 
parts  of  the  world.  The  spread  of  many  species  is  quite  obviously 
limited  by  the  prevailing  ecological  conditions.  Nevertheless,  the 
suitability  or  unsuitability  of  the  environment  is  not  alone  a 
sufficient  explanation  for  the  distribution  of  the  flora  and  fauna, 
for  introductions  have  shown  that  many  species  can  thrive  far 
beyond  the  limits  of  their  natural  range.  On  the  other  hand, 
within  a  given  land  mass,  even  though  a  variety  of  habitats 
exists,  the  species  tend  to  evidence  many  similarities  despite  their 
adaptation  to  different  conditions.  These  facts  are  most  easily  ex- 
plained by  the  theory  of  evolution.  Within  a  given  region  the 
variously  adapted  groups  have  evolved  from  a  common  ancestral 
stock;  hence  their  underlying  resemblances  that  made  possible  the 
identification  of  biogeographical  realms.  The  changing,  evolving 
species  in  one  area  can  only  spread  into  other  parts  of  the  world 
if  there  are  no  barriers  to  their  expansion.  Thus,  distribution  has 
an  historical  as  well  as  an  ecological  basis.  The  details  of  conti- 
nental, alpine,  and  island  distributions  of  living  species  have  be- 
come increasingly  well  understood  as  knowledge  of  paleontology 
and  past  geological  and  climatic  changes  has  increased.  Neverthe- 
less, the  theory  of  evolution  is  essential  to  a  complete  understand- 
ing of  present-day  distribution,  for  the  species  have  obviously 
been  dynamic  and  changing  rather  than  static  entities. 

SUGGESTED  READING 

Cain,  A.  S.,  1944.  Foundations  of  plant  geography.  New  York:  Harper. 
Darlington,  P.  J.,  1957.  Zoogeography.  New  York:  Wiley. 

Darwin,  C,  1839.  The  voyage  of  the  Beagle.  New  York:  Bantam  Books  (1958). 
Du  Toit,  A.  L.,  1937.  Our  wandering  continents.  Edinburgh:  Oliver  and  Boyd. 
Lack,  D.,  1947.  Darwin's  finches.  New  York:  Cambridge  University  Press. 
Matthew,  W.   D.,    1939.   Climate  and  evolution,   2d  ed.  New  York:   New  York 
Academy  of  Science. 


78  •  THE    EVIDENCE    FOR   EVOLUTION 

Simpson,  G.  G.,  1950.  "History  of  the  fauna  of  Latin  America,"  Amer.  Sci.,  38:361- 

389. 
,   1953.  Evolution  and  geography.  Eugene:  Oregon  State  System  of  Higher 

Education. 
Wallace,  A.  R.,   1876.  The  geographical  distribution  of  animals,  2  vols.  London: 

Macmillan. 

,  1911.  Island  life,  3d  ed.  London:  Macmillan. 

Wegener,  A.,  1924.  The  origin  of  the  continents  and  oceans,  3d  ed.  (J.  G.  A.  Skerl, 
tr.)  New  York:  Dutton. 


CHAPTER 


8 


Systematics 


Taxonomy  is  one  of  the  oldest  biological  disciplines,  but 
today  it  is  increasingly  being  pushed  into  the  background  by  the 
rapid  developments  in  such  fields  as  physiology,  ecology,  embry- 
ology, and  genetics.  Yet  taxonomy  remains  as  the  foundation 
stone  for  all  biological  research  simply  because  the  starting  point 
in  any  biological  experiment  is  an  organism,  and  in  order  to  con- 
duct and  describe  an  experiment  properly,  you  must  know  and 
know  with  certainty  what  organism  you  are  using.  Otherwise,  it 
may  be  impossible  for  you  or  anyone  else  to  confirm  or  to  dupli- 
cate your  results.  This  fact  has  all  too  often  been  slighted  or  over- 
looked, particularly  by  experimental  biologists,  who  may  speak 
of  using  "liver"  or  "frog  muscle"  as  if  all  livers  and  all  frog 
muscles  were  alike.  In  at  least  one  instance,  a  series  of  experi- 
ments was  abandoned  after  it  was  found  to  be  impossible  to 
identify  the  organisms  being  used. 

Classification 

All  of  us  are  taxonomists  to  some  extent,  in  that  we 
learn  to  identify  the  animals  and  plants  that  we  encounter  fre- 
quently. Taxonomy,  or  systematics  as  it  is  often  called,  grew  out 
of  the  study  of  local  faunas  and  floras.  As  information  accumu- 
lated, the  taxonomic  problems  quickly  became  more  complex  than 
those  encountered  in  a  local,  essentially  nondimensional  system. 
It  is  virtually  a  biological  axiom  that  no  two  organisms  are  iden- 
tical. Yet  it  is  also  true  that  some  organisms  are  much  more  alike 


79 


80  •  THE    EVIDENCE    FOR   EVOLUTION 

than  others.  The  taxonomist's  problem,  essentially,  is  to  seize  upon  the 
significant  similarities  and  thus  try  to  bring  some  sort  of  order  out  of 
this  chaos  of  variation.  Many  different  systems  are  possible.  Plants,  for 
example,  may  be  grouped  by  the  color  of  their  flowers  as  is  often  done  in 
popular  flower  guides,  or  by  their  habitats,  or  by  their  size,  and  so  on.  The 
method  used,  which  is  not  quite  so  simple,  is  known  as  the  "natural  system  of 
classification"  and  stems  from  Aristotle.  It  is  based  on  the  degree  of  similarity 
in  morphological  characters,  for  it  has  been  found  that  many  individuals  are 
very  much  alike  and  can  be  grouped  together  as  a  species.  All  house  cats,  for 
example,  belong  to  the  species  Felts  catus.  Certain  species,  in  turn,  are  quite 
similar  and  hence  are  grouped  together  in  a  higher  category,  the  genus.  The 
house  cat,  Veils  catus,  the  mountain  lion,  Felis  concolor,  and  the  lynx,  Felis  lynx, 
all  belong  to  the  genus  Felis.  Certain  genera  are  much  more  alike  than  other 
genera  and  thus  can  be  combined  into  a  family;  the  genus  ? anther  a,  which  in- 
cludes the  "big  cats"  such  as  lions,  tigers,  and  leopards,  together  with  the  genus 
Felts  belongs  to  the  family  Felidae.  The  family  Canidae  (dogs,  foxes,  and 
wolves),  the  Ursidae  (bears),  the  Mustelidae  (weasels,  skunks,  mink,  etc.),  the 
Felidae,  and  several  other  families  are  grouped  together  in  a  higher  group,  the 
order  Carnivora,  or  the  flesh  eaters.  The  orders  can  be  arranged  in  still  higher 
categories,  the  classes  and  phyla,  thus  forming  a  complete  hierarchy.  Each  family, 
for  example,  can  be  characterized  by  a  constellation  of  traits  that  sets  it  apart 
from  all  other  families  and  that  describes  not  only  each  genus  within  the  family, 
but  each  species,  and  even  each  individual.  Hence,  to  assign  a  species  to  a  par- 
ticular higher  group  characterizes  it  at  once  with  respect  to  a  certain  combination 
of  traits,  and  the  problems  of  dealing  with  over  a  million  different  species  are 
thereby  greatly  simplified.  Even  though  this  hierarchical  pattern  of  variation  was 
recognized  and  used  for  centuries,  it  remained  a  puzzle  as  to  why  organisms  fell 
into  this  particular  pattern  rather  than  some  other  geometrical  configuration. 

Variation 

At  this  point  it  is  well  to  consider  the  nature  of  variation  within  groups 
of  related  individuals.  First  of  all,  it  must  be  reemphasized  that  there  is  not  a 
continuum  in  the  pattern  of  variation.  There  are,  for  example,  no  individuals 
who  are  intermediate  in  their  traits  between  a  house  cat  and  a  dog.  Even  in  cases 
where  the  resemblance  is  much  closer  than  that  between  a  dog  arid  a  cat,  inter- 
mediates do  not  exist.  The  thrushes  of  the  genus  Hylocichla  are  very  difficult 
to  identify  in  the  field,  but  even  though  five  different  kinds — the  veery,  and  the 
wood,  hermit,  olive-backed,  and  gray-cheeked  thrushes — are  found  in  the  same 
region,  intermediate  types  will  not  be  found.  Without  now  attempting  a  species 
definition,  we  say  that  there  are  five  species,  each  composed  of  similar  but  not 
identical  individuals.  As  in  this  case,  species  are  for  the  most  part  quite  distinct 
from  each  other. 


SYSTEM ATICS  •  81 

Considerable  variation  may  exist  within  a  species,  for  within  a  given 
population  two  or  more  different  expressions  of  a  trait  may  appear,  a  type  of 
variability  called  polymorphism.  The  most  familiar  example  undoubtedly  is  a 
human  population  with  its  variety  of  sizes,  shapes,  eye  and  hair  colors,  and  so 
on  and  on,  but  populations  of  other  species  show  similar  variability.  Whether  it 
be  screech  owls,  deer  mice,  fruit  flies,  or  turtles,  variations  may  range  from  very 
minor  differences  to  such  a  striking  specimen  as  an  albino  snapping  turtle.  These 
differences  between  individuals  may  be  either  genetic  or  nongenetic  in  origin. 
Some  differences  are  simply  seasonal  or  age  differences.  The  spring  and  fall 
plumage  of  many  migratory  birds  and  the  differences  between  a  caterpillar  and 
a  butterfly  or  a  tadpole  and  a  frog  represent  merely  different  stages  in  the  life  of 
the  same  individual;  in  some  species  like  the  aphids,  seasonal  generations  exist. 
The  impact  of  the  environment  can  also  cause  wide  variations.  The  form  of 
corals  in  the  surf  is  quite  different  from  that  found  in  quiet  lagoons,  and  dande- 
lions growing  in  an  alpine  habitat  differ  in  form  from  those  in  the  valleys  below. 
The  hereditary  variations  include  the  differences  between  the  sexes,  which  may 
be  as  striking  as  the  presence  and  absence  of  wings  in  some  insects  or  antlers  in 
deer,  as  well  as  the  great  array  of  hereditary  variations  of  greater  or  lesser  degree 
to  be  found  in  all  sexually  reproducing  populations. 

Though  local  populations  are  polymorphic,  other  patterns  of  variation 
emerge  when  wider  areas  are  examined.  A  dine  is  said  to  exist  when  a  trait  or 
a  group  of  characters  is  observed  to  change  more  or  less  continually  and  gradu- 
ally as  one  moves  from  one  part  of  the  species'  range  to  another.  The  song 
sparrow,  Melospiza  melodia,  is  widely  distributed  and  common  in  North  Amer- 
ica but  is  by  no  means  uniform  throughout  its  range.  In  the  prairies  and  in  the 
arid  regions  of  the  Southwest  the  birds  are  paler  in  color;  in  the  more  humid 
regions  to  the  east  and  up  the  Pacific  coast  the  birds  are  duskier  in  color,  the 
transition  being  more  or  less  gradual  even  though  at  least  20  subspecies  have 
been  named.  Where  the  species  is  broken  up  into  more  clearly  defined  geograph- 
ical races  or  subspecies,  it  is  said  to  be  polytypic  (see  Fig.  8-1).  For  example,  in 
the  Philippines  a  small  kingfisher  inhabits  a  number  of  the  islands,  but  each 
island's  population  is  isolated  from  and  easily  distinguished  from  that  of  the 
other  islands.  Man,  too,  is  polytypic  as  well  as  polymorphic,  for  the  human 
species  is  readily  subdivided  into  three  major  geographic  races,  the  Negroid, 
Mongolian,  and  Caucasian. 

The  Binomial  System 

Modern  taxonomy  stems  from  the  1758  edition  of  Sy sterna  Naturae,  a 
volume  by  Linnaeus,  a  Swedish  botanist.  The  binomial  system  of  nomenclature 
that  he  introduced  was  simple  yet  precise — two  characteristics  needed  for  a 
workable  system.  For  example,  a  small  fish  can  easily  be  singled  out  if  it  is 
known    that   it   is   pale   brown    "with   a   dark   bar   behind    the   opercles   and 


82  •  THE    EVIDENCE    FOR   EVOLUTION 


Fig.  8-1.     The  bobwhite  quail,  a  polytypic  species.    Each  of  the  five  males,  shown 
in  dorsal  and  ventral  views,  is  representative  of  a  different  population  in  the  United 
States  or  Mexico.    All  five,  so  distinctive  in  appearance,  are  considered  to  be  mem- 
bers of  the  same  species,  Colinus  virginianus. 


SYSTEM ATICS  •  83 

across  the  dorsal  and  anal  fins,  which  are  bright  orange  in  spring  males.  The 
lips  are  thick  and  fleshy.  The  intestine  is  very  peculiar,  it  is  wrapped  many  times 
around  the  swim  bladder.  The  scales  are  7,  49-55,  8.  The  dorsal  fin  has  8  rays, 
the  anal  fin  7.  The  teeth  are  4-4.  This  species  reaches  a  length  of  8  inches." 
(Eddy  and  Surber)  Though  it  is  accurate,  no  one  in  his  right  mind  would  try 
to  use  this  description  in  everyday  conversation.  And  yet  the  common  name, 
stoneroller,  is  no  more  satisfactory,  for  what  is  one  man's  stoneroller  may  be 
called  stonelugger  by  another,  or  doughbelly,  or  even  rotgut  minnow.  The  more 
picturesque  common  names  suffer  from  their  lack  of  precision,  but  the  binomial, 
Campostoma  anomalum,  is  both  precise  and  brief,  and  has  been  assigned  to  the 
"minnows"  of  the  family  Cyprinidae  fitting  the  above  description. 

At  one  time  the  scientific  name  was  assigned  to  a  single  specimen,  the 
type  specimen,  and  all  individuals  collected  subsequently  were  referred  to  it  in 
order  to  determine  whether  they  belonged  to  the  same  or  a  different  species.  One 
of  the  major  advances  in  modern  systematics  is  that  the  type  concept  has  been 
almost  entirely  abandoned.  The  fallacy  of  the  type  concept  can  be  easily  made 
clear.  Suppose,  for  example,  you  were  told  to  go  out  and  collect  the  type  spec- 
imen for  the  species  Homo  sapiens.  Would  it  be  male,  or  female?  If  you  could 
settle  this  question  to  your  own  satisfaction,  how  would  you  then  decide  which 
member  of  your  sex  to  bring  in?  The  basic  facts  of  biological  variation  have 
made  it  abundantly  clear  that  the  type  specimen  is  not  typical  of  anything.  The 
important  point  to  determine  is  the  range  of  variation  in  the  species.  For  this 
purpose  adequate  sampling  methods  must  be  used  so  that  statistical  analyses  can 
be  applied.  Hence,  taxonomic  studies  are  becoming  studies  of  populations  rather 
than  of  individuals.  The  type  specimen  has  become  the  individual  to  which  the 
species  name  is  attached;  in  case  what  was  originally  thought  to  be  one  species 
later  turns  out  to  be  two,  the  original  name  will  be  reserved  for  individuals 
similar  to  the  type  and  a  new  name  assigned  to  the  other  group. 

As  mentioned  earlier,  the  natural  system  of  classification,  stemming 
from  Aristotle  and  formalized  by  Linneaus,  with  its  hierarchy  of  taxonomic 
groups  of  different  levels  of  morphological  similarity  was  always  something  of  a 
biological  puzzle  because  it  worked  so  well  even  though  there  was  no  obvious 
reason  why  this  particular  geometrical  configuration  should  exist  rather  than 
some  other.  The  publication  of  The  Origin  of  Species  in  1859  offered  a  simple 
solution  to  the  puzzle — that  is,  the  theory  of  evolution.  When  different  species 
are  similar,  the  similarities  are  due  to  descent  from  a  common  ancestry.  The 
closer  the  similarities,  the  more  recent  the  divergence  and  the  closer  the  genetic 
relationship  between  the  species.  After  Darwin,  the  natural  system,  based  on 
morphological  similarities,  became  a  phylogenetic  system  based  on  degree  of  re- 
lationship. It  might  be  expected  that  changing  the  criterion  for  classification 
would  drastically  change  the  classification  system  itself,  but  no  major  changes 
were  necessary.  Perhaps  the  main  inference  to  be  drawn  is  that  the  system  of 


84  •  THE    EVIDENCE    FOR   EVOLUTION 

classification  is  not  arbitrary  but  natural,  reflecting  the  objective  state  of  species 
in  nature.  And  systematics  has  become  more  than  classification;  it  has  become  the 
study  of  evolution. 

Some  Taxonomic  Problems 

Although  the  binomial  system  generally  works  beautifully,  anomalous 
situations  occasionally  arise  that  are  very  difficult  to  resolve  satisfactorily.  For 
example,  the  purple  grackle  breeds  in  a  belt  between  the  Appalachians  and  the 
Atlantic  from  just  north  of  New  Jersey  to  Florida  and  southern  Louisiana,  and 
the  bronzed  grackle  breeds  in  New  England  and  in  the  St.  Lawrence  and  Missis- 
sippi Valleys.  Yet  where  the  ranges  of  the  purple  and  bronzed  grackles  meet, 
all  along  the  Appalachians,  they  interbreed,  and  intermediate  types  of  individuals 
are  found.  At  present,  the  two  groups  are  considered  separate  species,  Quiscalus 
quiscula,  the  purple  grackle,  and  Quiscalus  versicolor,  the  bronzed  grackle. 
Where  such  extensive  interbreeding  occurs  over  such  a  large  area,  it  would 
seem  just  as  reasonable  to  consider  them  as  two  subspecies  of  the  same  species, 
which  replace  each  other  geographically. 

A  somewhat  different  situation  exists  in  the  leopard  frog,  Rana  pipiens, 
the  most  widely  distributed  frog  in  North  America,  ranging  from  Mexico  far 
into  Canada.  In  this  case  it  has  been  shown  that  when  frogs  collected  in  Florida 
or  Texas  are  crossed  with  those  from  Wisconsin  or  Vermont,  the  hybrids  are 
deformed  and  unviable.  In  other  words,  members  of  what  is  generally  regarded 
as  a  single  species  are  not  even  capable  of  interbreeding. 

One  further  instance  may  be  cited.  Butterflies  of  the  genus  ]unonia  are 
distributed  from  Florida  along  the  Gulf  Coast,  into  Mexico  and  Central  America, 
across  northern  South  America,  and  up  through  the  West  Indies  (see  Fig.  8-2). 
The  populations  gradually  change  in  their  characteristics  as  one  proceeds  around 
the  ring,  but  adjacent  populations  are  similar  and  are  capable  of  interbreeding. 
This  ring  of  races,  or  Rassenkreis  as  it  is  often  called,  is  closed  in  Cuba,  for 
there  butterflies  resembling  those  in  Florida  coexist  without  interbreeding  with 
butterflies  like  those  to  the  south  in  the  West  Indies.  In  Cuba,  then,  these  two 
populations  behave  like  two  distinct  and  well-defined  species,  yet  there  is  no 
single  place  around  the  ring  where  it  is  possible  to  say  that  here  one  species 
stops  and  the  other  begins. 

For  the  taxonomist  who  is  trying  to  work  out  a  satisfactory  scheme  of 
classification,  situations  such  as  the  three  cited  pose  very  real  and  very  tricky 
problems — and  there  are  many  others  even  more  complex.  For  the  student  of 
evolution,  however,  these  taxonomic  difficulties  furnish  still  another  argument  in 
favor  of  evolution.  If  evolution  is  a  gradual  process  that  has  been  in  progress 
through  time,  then  indications  that  species  are  now  undergoing  change  should 
be  expected  among  living  species.  The  existence  of  these  puzzling  taxonomic 


SYSTEM ATICS  •  85 


In  Cuba,  Northern 
and  Central  races 
coexist  without 
interbreeding 


Northern  race 


Fig.  8-2.     The  distribution   of  geographic  races  of  the  butterfly  Junonia  lavinia 
(Precis  lavinia)  commonly  known  as  the  Buckeye.    (Based  on  Forbes.) 


problems  is  evidence  that  species  are  not  static,  inflexible  units,  but  rather  are 
capable  of  change.  The  very  hierarchy  of  genera,  families,  orders,  and  so  forth  is 
in  itself  evidence  for  the  correctness  of  the  theory  of  evolution,  for  that  is  the 
pattern  that  evolution  should  cause  to  develop. 


86  •  THE    EVIDENCE    FOR   EVOLUTION 


SUMMARY  < 


At  first  acquaintance,  the  living  world  may  seem  a  chaos 
of  variation.  It  is,  however,  possible  to  bring  order  from  this 
chaos,  to  arrange  living  things  in  a  reasonable,  workable  system 
of  classification.  The  "natural  system  of  classification"  that  has 
developed,  culminating  in  the  Linnaean  binomial  system,  is  based 
on  the  degree  of  similarity  in  morphological  traits.  When  ar- 
ranged under  this  scheme,  living  things  fall  into  a  hierarchy  with 
the  similarities  becoming  more  specific  at  each  level  from  phylum 
to  genus.  The  theory  of  evolution  furnished  a  cogent  explanation 
for  this  pattern  of  variation.  The  similarities  so  readily  observed 
are  the  result  of  descent  from  a  common  ancestry  and  are  a  reflec- 
tion of  the  actual  genetic  relationship  between  the  species.  The 
taxonomically  difficult  groups  merely  confirm  the  theory  of  evolu- 
tion, for  the  difficulties  largely  arise  in  groups  that  are  in  the 
process  of  diverging  to  become  distinct  species — clear  evidence  of 
the  operation  of  evolution. 

SUGGESTED  READING 

Eddy,  S.,  and  T.  Surber,  1947.  Northern  fishes.  Minneapolis:  University  of  Minne- 
sota Press. 

Huxley,  J.,  ed.,  1940.  The  new  systematic s.  New  York:  Oxford  University  Press. 

Mayr,  E.,  1942.  Systematic s  and  the  origin  of  species.  New  York:  Columbia  Univer- 
sity Press. 

,  E.  G.  Linsley,  and  R.  L.  Usinger,  1953.  Methods  and  principles  of  system- 
atic zoology.  New  York:  McGraw-Hill. 


CHAPTER 


Comparative  Embryology 


Each  individual  starts  his  independent  existence  as  a 
single  cell,  the  fertilized  egg  or  zygote.  The  hereditary  material 
contained  by  each  zygote  is  the  surviving  product  of  millions  of 
years  of  evolution.  Each  zygote  develops  in  an  environment  of 
some  sort.  The  characteristics  of  the  adult  organism  are  deter- 
mined by  the  interaction  between  the  developing  embryo  and  its 
environment.  Abnormalities  either  in  the  transmitted  germ  plasm 
or  in  the  environment  may  cause  abnormal  development  in  the 
individual.  The  zygote  itself  is  a  spherical  object  bearing  little  or 
no  resemblance  to  the  adult  form,  which  is  only  reached  by 
gradual  stages.  The  sequence  of  stages  from  the  single  cell  to  the 
adult  and  beyond — that  is,  the  individual's  developmental  history 
from  fertilization  to  old  age — is  known  as  the  ontogeny  of  the 
individual.  The  various  adult  forms  of  an  evolving  species  may 
also  be  considered  as  a  series  of  stages  in  the  history  of  the 
species,  a  series  which  is  called  its  phytogeny.  With  two  such 
series  available,  it  was  almost  inevitable  that  someone  would  com- 
pare them.  Haeckel,  who  made  such  a  comparison,  propounded 
the  biogenetic  "law"  or  the  Theory  of  Recapitulation,  which 
states,  "Ontogeny  recapitulates  phylogeny."  In  other  words,  the 
embryo  in  its  development  retraces  its  evolutionary  path,  or 
climbs  its  family  tree  from  the  one-celled  ancestor  up  to  the 
present.  The  adult  stages  of  ancestral  forms  are  repeated,  but  they 
are  now  to  be  found  in  the  earlier  stages  of  ontogeny.  For  ex- 
ample, the  stage  early  in  development,  in  which  gill  slits  are  visi- 
ble in  birds  and  mammals,  was  considered  by  Haeckel  to  be  equiv- 


87 


88  •  THE   EVIDENCE    FOR   EVOLUTION 

alent  to  the  adult  fish  ancestors  in  the  phylogeny  of  these  groups.  Thus,  evolution 
was  thought  to  be  occurring  in  the  adult,  with  new  adult  forms  being  tacked  on 
to  the  old  at  the  end  of  the  developmental  period.  This  concept  has  had  con- 
siderable appeal,  especially  to  zoology  professors,  for  the  zygote  could  be  com- 
pared to  the  single-celled  protozoan  ancestor,  the  blastula  to  a  colonial  flagellate 
such  as  Volvox,  the  gastrula  stage  to  a  two-layered  coelenterate  like  Hydra,  and 
so  on.  Phylogeny  then  became  not  only  the  explanation  but  the  cause  of 
ontogeny,  a  conclusion  that  actually  hampered  research  into  the  causative  mechan- 
isms in  development. 

von  Baer's  Dicta 

Haeckel's  generalization  was  too  sweeping.  The  earlier  statements  of 
von  Baer,  though  less  striking,  were  more  accurate.  He  had  observed  that  in  de- 
velopment the  general  traits  appear  before  the  more  specialized,  that  the  embryos 
of  different  species  are  more  alike  than  the  adults  and  depart  progressively  from 
each  other  during  ontogeny,  and  that  the  young  stages  of  a  species  are  not  like 
the  adults  of  species  lower  in  the  phylogenetic  series  but  rather  like  their  embry- 
onic stages.  There  is  a  germ  of  truth  in  the  biogenetic  law  even  though  it  is 
demonstrably  false  if  taken  too  literally;  hence  it  would  be  more  proper  to  say, 
though  von  Baer  did  not,  that  "Ontogeny  recapitulates  ontogeny."  Vertebrate 
embryos  do  show  many  similarities,  for  which  the  most  reasonable  explanation  is 
their  common  ancestry. 

In  the  development  of  the  mammalian  heart,  for  example,  the  number 
of  chambers  is  initially  two,  then  three,  and  finally  in  the  adult,  four.  The  mam- 
malian phylogeny  includes  first  the  fishes  with  a  two-chambered  heart,  then  the 
amphibians  with  three,  and  the  reptiles  with  four.  The  basic  number  of  aortic 
arches  in  vertebrates  is  six,  the  living  fishes  having  arches  3  through  6  complete 
and  traces  of  the  first  two.  These  arches  break  up  into  capillary  beds  in  the  gills 
and  then  regroup  to  form  the  dorsal  aorta.  The  lower  amphibians  have  arches 
3  through  6,  but  the  lower  part  of  the  6th  aortic  arch  has  now  become  the 
pulmonary  artery  to  the  lungs.  In  the  higher  amphibians  and  reptiles  the  5th 
arch  is  also  missing  in  the  adult,  the  3rd  becomes  the  carotid  arteries  to  the 
head,  the  4th,  the  systemic  arteries  to  the  rest  of  the  body,  and  the  6th  remains 
pulmonary  in  function.  In  the  adult  mammals  only  the  3rd,  the  left  half  of  the 
4th  arch  (in  birds,  the  right  half),  and  the  lower  part  of  the  6th  are  all  that 
remain  functional  of  the  six  arches  that  make  their  transient  appearance  during 
development.  (See  Fig.  9-1). 

Man's  evolutionary  past  sometimes  manifests  itself  in  strange  ways. 
From  time  to  time  we  read  of  so-called  "blue  babies,"  who  are  suffering  from 
insufficient  oxygenation  of  their  blood.  There  are  two  major  causes  for  this  con- 
dition: either  the  opening  between  the  right  and  left  auricles  of  the  heart  does 


COMPARATIVE    EMBRYOLOGY  •  89 

Carotid 


Ventral  aorta 
■Dorsal  aorta 

HYPOTHETICAL  SHARK 

ANCESTRAL  TYPE 


Pulmonary 
artery 


FISH 
(Teleost) 


AMPHIBIAN 
(Urodele) 


AMPHIBIAN 
(Anuran) 


REPTILE 


BIRD 


OJl 
MAL 


Fig.  9-1.     Diagram  of  the  evolution  of  the  aortic  arches  in  the  vertebrates  (ventral 

views). 


not  close,  or  the  duct  of  Botallus,  a  vessel  connecting  the  pulmonary  artery 
directly  to  the  dorsal  aorta,  fails  to  close.  Both  opening  and  duct  are  devices  by 
which  the  blood  of  the  fetus  is  shunted  past  the  nonfunctional  lungs  prior  to 
birth.  Since  the  opening  between  the  auricles  represents  a  persistence  of  the 
ancestral  two-chambered  fish  heart  and  the  duct  of  Botallus  is  actually  the  upper 
half  of  the  6th  aortic  arch,  these  blue  babies  are  living  evidence  of  man's  evolu- 
tionary past. 


90  •  THE    EVIDENCE    FOR   EVOLUTION 

The  gill  arches  and  the  gill  slits  in  the  mammalian  embryos  do  not 
represent  the  adult  ancestral  fish,  but  are  similar  to  those  of  a  fish  embryo  at  a 
comparable  stage  of  development.  They  then  differentiate  into  structures  quite 
different  from  those  in  the  fish.  All  of  the  gill  slits  close  and  disappear  except 
the  one  that  forms  the  Eustachian  tube,  which  connects  the  pharynx  at  the  back 
of  the  mouth  to  the  middle  ear.  The  gill  arches  themselves  have  a  variety  of 
fates.  In  the  most  primitive  jawless  fishes,  of  which  the  lamprey  is  a  surviving 
relict,  the  gill  arches  number  seven.  The  first  arch  became  the  basis  for  the  jaws 
in  the  fishes,  but  the  bones  forming  the  jaw  articulation  in  fishes,  the  quadrate 
and  the  articular,  by  an  unusual  turn  of  events  have  moved  into  the  middle  ear 
of  the  mammals  during  the  course  of  evolution.  There,  as  the  incus  (or  anvil, 
formerly  the  quadrate)  and  the  malleus  (or  hammer,  formerly  the  articular), 
they  form  two  thirds  of  the  chain  of  small  bones  that  conduct  sound  across  the 
middle  ear  to  the  inner  ear.  The  third  bone  in  this  chain,  the  stapes  or  stirrup,  is 
derived  from  the  second  gill  arch,  which  as  the  hyomandibular  in  fish  more  or 
less  anchors  the  jaws  to  the  brain  case.  The  rest  of  the  2nd  gill  arch  forms  the 
body  and  the  anterior  horn  of  the  hyoid  apparatus,  the  posterior  horn  coming 
from  the  3rd  gill  arch.  The  hyoid  apparatus  and  other  cartilaginous  structures  in 
the  throat  region  such  as  the  thyroid,  arytenoid,  and  cricoid  cartilages,  derived 
from  the  4th  and  5th  arches,  are  relatively  insignificant  compared  to  their  size 
and  functional  importance  in  fish.  (See  Fig.  9-2.)  All  of  the  above  statements 
are  well  grounded  on  embryological  and  anatomical  evidence.  The  obvious  ques- 
tion is  why  there  should  be  a  stage  in  the  mammalian  embryo  where  gills  and 
gill  arches,  which  never  function  as  such,  are  nevertheless  present,  even  though 
they  differentiate  into  quite  different  adult  structures.  The  most  obvious  answer 
is  that  the  mammals  are  descended  from  fishlike  ancestors  and  that  in  the  course 
of  evolution  modifications  in  development  have  occurred;  the  similarities  which 
still  persist  in  the  ontogeny  of  fish  and  mammals  are  indicative  of  a  funda- 
mental similarity  in  their  genotypes  due  to  their  common  ancestry. 

Modifications  of  Development 

The  notochord,  characteristic  of  the  Phylum  Chordata,  to  which  the 
vertebrates  belong,  is  crowded  out  by  the  vertebrae  almost  as  soon  as  it  is  formed 
in  the  vertebrate  embryo.  Why,  then,  is  the  notochord  retained?  It  might  seem 
to  be  a  clear-cut  case  of  recapitulation,  but  this  can  hardly  be  so.  The  cells  that 
form  the  notochord  are  intimately  bound  up  with  the  organizing  and  inducing 
of  the  essential  axial  structures  of  the  embryo — the  spinal  cord  and  brain,  the 
heart,  kidneys,  muscle,  and  so  on;  thus  if  this  function  is  to  be  retained,  the 
cells  themselves  must  be  retained.  Because  natural  selection  acts  on  living  organ- 
isms at  all  stages  of  their  existence,  not  just  upon  the  adults,  embryonic  as  well 
as  adult  stages  and  structures  may  be  changed,  added,  or  eliminated.  Since  selec- 
tion must  act  within  the  limits  imposed  by  the  modifications  possible  in  already 
existing  stages,  the  retention  of  stages  similar  to  those  of  ancestral  forms  is  to  be 


COMPARATIVE    EMBRYOLOGY  •  91 


Gill  slit     Brain  case 


Hyomandibular 
Palaroquadrate 


*"*••—.?.... 4       5       6      7 
PRIMITIVE  JAWLESS  TYPE 


Meckel's 
cartilage 


SHARK 


^•-    Columella  (stapes) 
Quadrate 


AMPHIBIAN 
OR  REPTILE 


Gill  arch  1 

(upper  jaw  of  shark, 

palatoquadrate) 

Gill  arch  1 

(lower  jaw  of  shark, 

Meckel's  cartilage) 

Gill  arch  2 
(hyomandibular  of 
shark;  hyoid) 


Gill  arches  3-7 


Thyroid  cartilage 
Cricoid  cartilage 


Tracheal  cartilages- 

Fig.  9-2.     Evolution  of  the  gill  arches  in  vertebrates. 


Styloid  process 
of  hyoid 
Meckel's  cartilage 

—  Ligaments 

Hyoid 

Larynx 
OJI 

HUMAN 


expected  even  though  their  subsequent  developmental  fates  may  differ.  Many 
kinds  of  modifications  of  developmental  patterns  may  be  observed. 

In  the  typical  frog,  for  example,  the  small  eggs  laid  in  water  hatch  after 
a  few  days  into  free-living,  gill-breathing  tadpoles  that  metamorphose  after 
several  weeks  or  months — or  even  years,  in  the  bullfrog — into  the  adult  frog. 
In  the  Hylodes  of  the  West  Indies,  however,  the  large  eggs  laid  on  leaves  hatch 
in  two  or  three  weeks  directly  into  frogs,  although  a  brief  tadpole  stage  exists 


92  •  THE    EVIDENCE    FOR   EVOLUTION 

prior  to  the  hatching  of  the  frogs  from  the  eggs.  The  elimination  of  the  func- 
tional tadpole  stage  has  taken  place,  but  the  tadpole  nevertheless  continues  to 
appear;  thus,  although  a  secondary  modification  of  the  basic  plan  of  frog  devel- 
opment has  occurred,  the  change  has  not  been  sufficiently  drastic  to  eliminate  the 
stage  completely.  Such  information  is  evidence  not  only  for  evolution,  but  for 
its  gradual  nature. 

The  fossil  evidence  and  other  evidence  make  it  abundantly  clear  that  the 
Amphibia  are  ancestral  to  the  reptiles,  birds,  and  mammals.  The  three  latter 
groups  are  known  as  the  amniotes,  for  their  embryos  develop  within  the  watery 
cradle  made  possible  by  embryonic  membranes  known  as  the  amnion  and 
chorion.  Yet  since  the  amphibians  lack  these  membranes,  they  must  be  new 
structures  evolved  during  the  evolution  from  amphibians  to  reptiles.  In  the 
mammals,  a  modification  in  function  led  to  the  utilization  of  the  chorion  as  a 
part  of  the  placenta.  Thus  new  structures  or  modification  of  existing  structures 
for  new  functions  can  evolve  in  the  embryo  as  well  as  in  the  adult. 

In  some  instances  precocious  sexual  maturity  has  led  to  the  elimination 
of  the  adult  stage,  a  phenomenon  known  as  paedogenesis.  In  the  axolotls, 
salamanders  of  the  genus  Ambystoma  having  a  gill-breathing,  water-dwelling 
larval  stage,  the  larvae  may  mature  sexually  and  reproduce  without  undergoing 
metamorphosis.  That  this  is  an  example  of  paedogenesis  is  proved  by  the  fact 
that  the  axolotl,  under  certain  environmental  conditions,  metamorphoses  into  the 
adult  lung-breathing,  land-dwelling  form.  Compared  to  the  other  primates,  man 
has  an  extended  developmental  period;  in  fact,  human  adults  show  more  resem- 
blance to  immature  anthropoids  than  to  the  adult  great  apes.  The  lack  of  hair 
and  of  well-developed  brow  ridges,  the  relatively  flat  face,  and  the  slow  closure 
of  the  skull  sutures  have  all  been  singled  out  as  indicative  of  a  tendency  toward 
paedogenesis  in  man. 

New  and  different  stages  in  the  life  cycle  have  also  evolved.  Among  the 
primitive  insects,  the  immature  forms  are  rather  similar  in  appearance  and  func- 
tion to  the  adults  or  imagoes.  In  the  more  recent  groups  of  insects,  the  egg 
hatches  into  a  larva  quite  different  in  form,  function,  and,  usually,  habitat  from 
the  adult  into  which  it  later  metamorphoses.  The  caterpillars  that  become  butter- 
flies and  the  squirming  maggots  that,  after  a  quiescent  pupal  stage,  emerge  as 
flies,  are  familiar  examples  of  insect  metamorphosis.  An  example  can  even  be 
cited  much  like  Haeckel's  concept  of  evolution:  in  the  development  of  the  crab, 
the  megalopa  stage  resembles  a  lobster  or  crayfish,  near  relatives  of  the  crabs, 
and  the  adult  crab,  with  abdomen  folded  under,  is  a  stage  that  appears  to  be 
tacked  on  to  the  ancestral  form. 

Thus,  it  is  clear  that  many  changes  in  ontogeny  have  occurred:  new 
embryonic  stages  not  affecting  the  adults,  for  example,  parasitic  larvae  of  free- 
living  adults;  wide  divergence  of  adults  with  similar  embryos,  for  example,  fish 
and  mammalian  embryos;  adult  forms  that  may  resemble  larval  stages  of  ances- 
tors, that  is,  paedogenesis;  or  appearance  of  a  new  adult  stage  apparently  added 


COMPARATIVE    EMBRYOLOGY  •  93 

to  the  previous  adult  stage.  These  changes  must  be  due  to  the  action  of  natural 
selection,  producing  changes  in  relative  rates  of  development  of  various  struc- 
tures as  well  as  modifications  in  the  function  and  structure  of  existing  stages  and 
structures.  Where  repetition  of  ancestral  stages  occurs,  it  is  not  simply  a  case  of 
Haeckelian  recapitulation,  but  rather  an  indication  that  similar  groups  of  genes 
are  operative  and  that  the  embryonic  structures  they  control  are  still  essential  to 
normal  ontogeny,  and  hence  have  not  been  eliminated  by  natural  selection. 
Therefore,  the  study  of  embryology  is  helpful  in  determining  relationships,  and 
the  rejection  of  Haeckel's  dictum  does  not  imply  a  rejection  of  all  embryological 
evidence  relating  to  evolution,  for  similarities  in  ontogeny  are  often  indicative 
of  phylogenetic  relationship.  In  fact,  they  may  often  be  the  best  evidence  avail- 
able. In  the  free-living  shrimp  (Penaeus),  the  sessile  barnacle  (Lepas),  and 
Sacculina,  a  parasitic  sac  in  the  crab,  the  Nauplius  larval  form  of  all  three  is  the 
best  evidence  that  these  three  diverse  adult  types  are  members  of  the  Crustacea. 
Here  and  in  many  other  instances,  similarity  in  ontogeny  is  an  indication  of 
genetic  affinity  but  is  not  necessarily  evidence  as  to  the  adult  form  of  the 
ancestors. 


►  SUMMARY 


Despite  the  diversity  of  form  among  such  groups  as  fish, 
amphibians,  reptiles,  birds,  and  mammals,  the  embryos  of  all  of 
these  vertebrates  look  very  similar  and  have  many  features  such  as 
gill  slits,  aortic  arches,  neural  tube,  and  notochord  in  common. 
Thus,  the  adult  diversity  results  from  the  modification  during  de- 
velopment of  the  same  basic  embryonic  plan.  The  assumption  that 
these  groups  are  all  descended  with  modification  from  a  common 
fish  ancestry  renders  this  situation  intelligible.  Other  theories  are 
quite  inadequate  to  account,  for  example,  for  the  presence  of  gill 
slits  in  birds  and  mammals,  which  never  at  any  stage  in  their  life 
cycle  require  functional  gills.  The  recapitulation  theory  of 
Haeckel,  as  originally  stated,  represents  an  oversimplification  of 
the  facts,  for  the  developing  embryo  does  not  recapitulate  the  adult 
stages  of  its  ancestors.  Rather,  the  embryo  will  in  most  instances 
show  more  resemblance  to  the  embryos  of  ancestral  or  related 
groups  than  it  will  to  their  adult  forms.  For  this  reason  compara- 
tive embryology  can  be  a  fruitful  source  of  phylogenetic  informa- 
tion. The  evidence  indicates  that  evolution  must  operate  within 
the  framework  and  limitations  imposed  by  existing  patterns  of 
development.  Although  the  end  products  in  some  cases  have  been 
as  diverse  as  a  fish  darting  through  the  water  and  a  bird  soaring 
in  the  sky,  their  embryos  still  carry  the  clues  to  their  common 
ancestry. 


94  •  THE    EVIDENCE    FOR   EVOLUTION 

SUGGESTED  READING 

DeBeer,  G.  R.,  1958.  Embryos  and  ancestors,  3d  ed.  New  York:  Oxford  University 
Press. 

Nelsen,  O.  E.,  1953.  Comparative  embryology  of  the  vertebrates.  New  York: 
Blakiston. 

Willier,  B.  H.,  P.  A.  Weiss,  and  V.  Hamburger,  eds.,  1955.  Analysis  of  develop- 
ment. Philadelphia:  Saunders. 


CHAPTER 


10 


Comparative  Anatomy 


The  similarity  between  different  species  was  one  of  the 
fundamental  reasons  for  the  development  of  the  theory  of  evolu- 
tion, and  comparative  anatomy  has  been  one  of  the  cornerstones 
of  evidence  for  the  theory  ever  since  Darwin's  time.  In  a  sense, 
comparative  embryology  and  comparative  anatomy  are  one  and 
the  same  study,  differing  only  with  respect  to  the  stage  of  devel- 
opment of  the  organism,  but  historically  and  traditionally  two 
disciplines  have  existed  rather  than  one.  Unfortunately,  not  all 
similarities  between  members  of  different  species  are  due  to  a 
common  ancestry,  and  the  concept  has  sometimes  been  consider- 
ably overworked.  Lamarck  and  especially  St.  Hilaire  argued  that 
all  animal  species  conformed  to  a  common  archetype,  a  clearly 
erroneous  idea  that  was  strongly  and  effectively  attacked  by 
Cuvier.  The  fallacy  of  the  archetype  concept  can  be  seen  through 
a  comparison  of  such  "higher"  animals  as  a  mammal,  an  insect, 
and  a  mollusk  like  the  snail;  neither  in  general  nor  in  particulars 
can  they  be  truly  said  to  conform  to  a  common  pattern  at  any 
stage.  Lamarck's  adherence  to  this  concept  undoubtedly  weakened 
his  arguments  for  evolution  and  may  well  be  responsible  for  the 
fact  that  we  now  associate  the  theory  of  evolution  with  Darwin 
rather  than  Lamarck. 

Homology  and  Analogy 

There  are  apparently  two  major  reasons  for  similarities 
between  species — heritage  and  habitus.  Heritage  refers  to  a  com- 


95 


96  •  THE    EVIDENCE    FOR   EVOLUTION 

mon  ancestry,  with  similar  genetic  systems  responsible  for  the  resemblances. 
However,  species  with  similar  modes  of  life  are  often  very  much  alike  even 
though  not  closely  related.  The  mechanism  responsible  for  this  type  of  simi- 
larity is  natural  selection,  similar  selection  pressures  bringing  about  similar 
adaptations  to  similar  environments.  The  problem,  of  course,  is  to  be  sure  that 
relationships  attributed  to  heritage  are  not  actually  due  to  habitus,  a  distinction 
not  always  easily  made.  Two  concepts  have  arisen  in  connection  with  these 


r :  -\ 


TkWtott 


Fig.  10-1.     Analogy.    (From  Animal  Analogues  by  R.  W.  Wood.) 


differences  that  aid  in  clarifying  the  ideas  involved;  structures  that  are  similar 
because  of  similar  function  or  habitus  are  said  to  be  analogous,  whereas  struc- 
tures that  are  similar  because  of  common  ancestry  and  a  similar  genetic  basis  are 
said  to  be  homologous. 

The  wings  of  a  swallow  and  a  dragonfly,  though  used  by  both  in  flight, 
are  analogous  since  their  origin  and  structure  are  clearly  different.  The  fins  of  a 
trout  and  a  dytiscid  water  beetle  are  also  analogous.  In  both  of  these  examples 
the  structural  differences  between  the  vertebrate  and  the  insect  are  fairly  obvious, 
but  this  is  not  always  the  case.  The  camera-type  eye  with  a  focusing  lens  and  a 
sensitive  pigment  layer  has  appeared  in  two  groups  of  animals,  the  vertebrates 
and  the  cephalopod  mollusks  such  as  the  squid  and  the  octopus.  The  physical 
requirements  for  this  type  of  eye  are  such  that  they  must  be  quite  similar  struc- 


COMPARATIVE    ANATOMY  •  97 

turally  if  the  eye  is  to  function  at  all.  Both  have  a  lens,  a  sensitive  pigment 
layer,  and  a  layer  of  nerves,  all  housed  in  a  spherical  chamber,  and  superficially 
are  much  alike.  However,  the  embryology  of  the  eye  in  the  two  groups  is  quite 
different.  Most  striking,  perhaps,  is  the  fact  that  the  vertebrate  eye  is,  in  a  sense, 
arranged  backward;  that  is,  the  layer  of  nerves  carrying  the  impulses  to  the  brain 
lies  in  front  of  the  pigment  layer  rather  than  behind  it,  the  latter  being  a  more 
sensible  arrangement  and  the  one  that  is  found  in  the  cephalopod  eye.  It  is  clear 
from  these  examples  that  a  similar  problem,  whether  it  be  flying,  swimming,  or 
seeing,  is  apt  to  have  similar  solutions  in  different  groups.  Even  though,  at  the 
outset,  the  heredity  may  be  very  different,  the  end  products  of  the  operation  of 
natural  selection  are  much  alike.  The  evolution  of  widely  divergent  groups  to- 
ward greater  similarity  due  to  common  functions  or  adaptations  is  known  as 
convergent  evolution.  The  resemblances,  however,  are  always  superficial. 

Homologous  structures,  on  the  other  hand,  may  or  may  not  function 
alike;  homology  rests  not  on  function  but  on  a  similar  developmental  origin  and 
hereditary  basis.  A  human  hand,  a  bat's  wing,  and  a  cat's  forepaw,  for  example, 
are  homologous,  for  all  are  five-toed  (pentadactyl)  structures,  functionally  quite 
different,  but  of  similar  location  and  embryology  in  three  different  mammals. 

The  distinction  between  homology  and  analogy  may  seem  relatively 
clear-cut,  but  cases  do  arise  where  the  decision  will  depend  on  point  of  view 
rather  than  any  fixed  criterion.  The  wing  of  a  bird,  the  wing  of  a  bat  (a  mam- 
mal), and  the  wing  of  a  pterosaur  (a  flying  reptile)  are  all  derived  from  the 
vertebrate  tetrapod  forelimb  and  are  thus  homologous,  in  one  sense.  However, 
flight  originated  independently  in  these  three  groups,  and  the  three  types  of 
wings  are  quite  different  in  the  details  of  their  structure.  In  the  bat  wing  all  five 
digits  of  the  pentadactyl  forelimb  are  present.  The  wing  of  a  bird  utilizes  only 
digits  1,  2,  and  3,  and  in  quite  a  different  manner,  with  the  fourth  and  fifth 
digits  completely  lost.  The  pterodactyl  had  four  digits,  with  only  the  fourth 
elongated  to  support  the  wing  and  the  fifth  missing  (see  Fig.  10-2).  With  re- 
spect to  their  adaptations  for  flight,  then,  these  wings  should  more  properly  be 
regarded  as  analogous  rather  than  homologous. 

Homologies  in  Vertebrates 

Obviously,  it  is  not  possible  to  explore  in  detail  the  great  wealth  of 
material  on  comparative  anatomy  that  has  been  amassed  for  many  different 
groups.  Volumes  have  been  written  even  for  a  single  group  such  as  the  verte- 
brates (see  references  at  end  of  chapter) .  Careful  study  of  these  texts  and  first- 
hand experience  with  the  organisms  themselves  give  an  extremely  convincing 
demonstration  of  the  reality  of  evolution.  However,  some  selected  examples  will 
serve  to  illustrate  the  nature  of  this  type  of  evidence. 

Characteristically  there  are  seven  cervical  vertebrae  in  the  mammalian 


98  •  THE    EVIDENCE    FOR   EVOLUTION 

neck;  a  mouse,  an  elephant,  and  even  a  giraffe  have  the  same  number  of  cervical 
vertebrae.  These  mammals  have  a  defined  neck  region  and  are  capable  of  turning 
their  heads,  whereas  the  porpoise,  a  mammal  with  the  torpedolike  shape  charac- 
teristic of  the  fishes,  lacks  a  distinguishable  neck  region  and  cannot  turn  its  head. 
Nevertheless,  the  seven  cervical  vertebrae  are  present  in  the  porpoise  although 
they  are  much  shorter  than  in  mammals  of  comparable  size  and  are  fused  to- 


Fig.  10-2.     Homology  in  vertebrate  wings. 


gether  so  that  flexibility  has  been  lost.  To  the  obvious  question  as  to  why  animals 
differing  so  greatly  in  size,  in  structure,  and  in  mode  of  life  should  have  the 
same  number  of  vertebrae  in  their  necks,  the  theory  of  evolution  presents  a 
simple,  plausible  answer.  All  these  varied  forms,  and  the  many  other  mammals, 
are  descended,  with  modifications,  from  an  ancestral  mammalian  stock  that  was 
characterized  by  seven  cervical  vertebrae. 

The  evolution  of  the  vertebrate  skull,  in  which  homologies  have  been 
traced  from  the  fish  up  through  the  amphibians  and  the  reptiles  to  the  present- 


COMPARATIVE    ANATOMY  •  99 

day  mammals,  illustrates  the  amount  of  change  that  has  taken  place  in  the  many 
millions  of  years  of  vertebrate  history.  The  mammalian  skull,  an  apparently  uni- 
tary structure,  has  been  shown  to  have  been  formed  from  three  quite  distinct 
components  found  in  the  fish  skeleton:  the  endoskeletal  brain  case,  the  dermal 
bony  armor  in  the  head  region,  and  the  visceral  skeleton  supporting  the  gill 
arches  (see  Fig.  10-3).  The  original  braincase  housed  the  major  sense  organs — 


MAN 


Nasal  =  N 

Maxillary  =M 

Dentary  =  D 

Frontal  =  F 

Parietal  =  P 

Jugal =  J 

Temporal  =  7 

Occipital  =0 


LION 


Fig.  10-3.     Homology  in  the  bones  of  the  skull. 


of  sight,  hearing,  and  olfaction — and  was  shielded  by  a  complete  roof  of  dermal 
bones  imbedded  in  the  skin.  The  jaws  were  originally  derived  from  the  gill 
arches.  By  a  series  of  extensive  changes  involving  modification,  fusion,  or  loss 
of  the  bones  in  the  fish  skull,  the  mammalian  skull  such  as  that  of  the  cat  has 
arisen.  Although  the  homologies  between  the  fish  and  cat  skull  are  by  no  means 
obvious  without  adequate  study  of  the  many  forms  representative  of  the  numer- 
ous intermediate  stages,  and  many  people  find  it  difficult  in  any  event  to  accept 
that  modern  man's  gum-chewing  jaws  are  derived  from  structures  that  originally 


100  •  THE    EVIDENCE    FOR   EVOLUTION 

supported  the  gills  of  fish,  the  homologies  between  the  cat  (Felts  catus)  and  the 
lion  (Panthera  leo)  skulls  are  quite  clear.  The  homologies  are  not  so  obvious 
between  these  skulls  and  that  of  man,  in  a  different  mammalian  order,  but  study 
of  the  diagrams  will  show  the  many  similarities  between  them. 

The  pentadactyl  appendage  has  already  been  mentioned  as  the  character- 
istic condition  in  tetrapods,  but  not  all  tetrapods  have  five  toes  on  each  ap- 
pendage, and  it  may  be  questioned  whether  some  of  them  ever  did  have  five 
toes.  In  addition  to  the  embryological  evidence  and  the  vestiges  of  digits  that 
indicate  the  previous  presence  of  additional  digits,  another  type  of  evidence, 
from  guinea  pigs,  is  now  available.  The  guinea  pig  has  four  toes  on  each  fore- 
foot, but  only  three  on  each  hind  foot;  a  hereditary  variant,  called  pollex,  has 
been  discovered  that  produces  the  five-toed  condition  on  all  four  feet.  Though  it 
could  be  argued  that  such  a  mutation  has  no  evolutionary  significance,  it  seems 
more  reasonable  to  suppose  that  it  has  restored  the  ancestral  condition,  and  in 
any  case  it  certainly  establishes  that  guinea  pigs  can  have  five  toes. 

Genetic  Homology 

Morphological  homologies  are  actually  based  on  homologies  in  the 
hereditary  materials  or  genotypes  of  different  species,  of  which  they  are  the  most 
obvious  manifestations.  It  is  therefore  significant  that  when  it  has  been  possible 
to  study  genetic  homologies  more  directly,  homologous  genes  have  been  demon- 
strated in  closely  related  species.  In  different  species  of  flies  of  the  genus 
Drosophila,  similar  mutations  affecting  eye  color,  body  color,  the  bristles,  and 
other  traits  have  been  shown  to  exist.  The  homologies  have  been  based  not  only 
on  the  similarities  in  phenotype,  but  on  the  location  of  these  genes  in  homo- 
logous regions  of  the  chromosomes  and  in  some  cases  by  crosses  as  well. 

Serial  homology  is  a  somewhat  different  concept  from  the  one  we  have 
been  considering,  but  it,  too,  has  evolutionary  significance.  The  segmented  ani- 
mals such  as  the  vertebrates  and  the  arthropods  are  composed  of  a  series  of 
segments,  each  of  which  is  basically  similar  to  the  others,  and  the  structures  in 
one  segment  can  be  compared  and  homologized  to  those  in  other  segments.  Serial 
homologies  are  clear-cut  in  an  animal  like  the  earthworm,  an  annelid,  where 
most  of  the  segments  are  replicas  of  each  other.  Even  in  arthropods  such  as  the 
lobster  and  crayfish  in  which  considerable  differentiation  of  the  segments  has 
occurred,  the  homologies  between  various  appendages  such  as  the  mandibles,  the 
legs,  the  claws,  and  the  antennae  are  easy  to  visualize.  The  segmentation  of  many 
insect  larvae  shows  relatively  little  differentiation,  and  the  homologies  are  there- 
fore easily  established;  but  in  adult  insects,  the  great  degree  of  differentiation 
serves  to  mask  not  only  the  homologies  but  even  the  segmentation  itself  to  some 
extent.  Nevertheless,  in  the  insects  the  mouth  parts,  the  antennae,  and  the  legs 
have  been  considered  to  be  serially  homologous  despite  their  dissimilarity  in 


COMPARATIVE    ANATOMY  •   101 

appearance  and  function.  The  discovery  of  the  so-called  homeotic  mutants  in 
Drosophila  has  tended  to  reinforce  these  conclusions.  The  aristapedia  mutant 
causes  the  development  of  a  leglike  structure  in  place  of  the  antenna,  and 
proboscipedia  causes  a  similar  change  in  the  proboscis.  Thus,  the  homeotic  mu- 
tants cause  one  of  a  series  of  parts  to  assume  the  character  of  another  member  of 
the  series,  and  by  demonstrating  the  common  potentialities  of  these  varied  ap- 
pendages have  tended  to  confirm  the  conclusions  previously  drawn. 

In  mo|t  orders  of  insects  there  are  two  pairs  of  wings  located  on  the 
second  and  third  thoracic  segments.  In  the  two-winged  flies  of  the  order  Diptera, 
the  second  segment  bears  the  single  pair  of  wings  and  the  third  bears  the 
halteres,  a  pair  of  gyroscopic  devices.  The  inference  that  the  halteres  are  homo- 
logous (and  serially  homologous)  to  wings  has  been  strengthened  by  the  dis- 
covery of  the  homeotic  mutants  tetraptera,  which  produces  a  four-winged 
dipteran,  and  tetraltera,  which  causes  flies  with  four  halteres  to  develop.  The 
discovery  of  mutants  that  change  the  ordinal  characters  of  individuals  carrying 
them  has  led  some  students,  notably  Goldschmidt,  to  believe  that  the  higher 
taxonomic  groups  have  originated  in  this  fashion,  an  interesting  speculation  that 
does  not  appear,  however,  to  be  borne  out  by  the  facts. 

Vestigial  Organs 

Another  type  of  evidence  for  evolution  is  derived  from  the  so-called 
vestigial  structures.  Not  only  do  they  suggest  relationships,  but  they  also  raise 
questions  about  the  mechanism  of  evolution;  many  vestigial  organs  have  lost 
their  adaptive  function,  and  it  may  well  be  asked  why  they  should  continue  to 
persist.  Man  himself  is  virtually  a  walking  museum  from  his  head  to  his  feet. 
Many  people,  for  example,  have  small  nodes  on  their  ears,  known  as  Darwin's 
points,  which  are  thought  to  be  vestiges  of  the  somewhat  larger  and  more 
pointed  ears  of  our  ancestors.  And  even  though  we  can  no  longer  rotate  our  ears 
to  test  the  sounds  carried  by  each  vagrant  breeze  as  do  the  deer,  nevertheless 
vestiges  of  these  muscles  remain  that  permit  small  boys  and  gentlemen  at  parties 
to  show  off  by  wiggling  their  ears.  Human  facial  contortions  are  controlled  by 
the  remnants  of  the  muscles  with  which  our  remote  fish  ancestors  aerated  their 
gills.  When  cold,  our  mammalian  relatives  fluff  out  their  fur  to  increase  the 
insulation  of  their  bodies;  we  get  goose  pimples  or  duck  bumps  under  the  same 
conditions,  but  the  attempt  is  abortive,  for  even  though  the  muscles  for  fluffing 
the  hair  are  present,  the  hair  itself  has  virtually  no  insulating  capacity.  When 
angry  or  excited  or  frightened,  your  dog  may  raise  the  hackles  along  his  neck, 
something  we  also  try  to  do  when  we  get  the  "chills"  in  a  horror  movie.  The 
appendix  and  the  coccyx  are  classical  examples  of  human  vestigial  organs.  The 
coccyx  is  all  that  remains  of  our  tail,  and  the  appendix  seems  to  be  of  more 
trouble  than  value  as  an  adjunct  to  the  human  intestine.  Even  the  human  foot- 


102  •  THE    EVIDENCE    FOR   EVOLUTION 

print,  showing  the  arch  and  the  big  first  toe,  is  a  vestige  of  our  simian  ancestry 
and  our  former  habitat  in  the  trees. 

The  theory  of  evolution  gives  a  simple  explanation  for  the  presence  of 
vestigial  structures.  The  presence  of  a  pelvic  girdle  in  the  python  and  the  whale, 
a  reptile  and  a  mammal  respectively,  neither  of  which  has  hind  limbs,  is  clear 
evidence  that  they  are  descended  from  tetrapod  ancestors.  Any  other  explanation 
is  extremely  difficult  to  apply  or  to  accept. 


SUMMARY  <■ 


Comparative  anatomy  rests  on  the  distinction  between 
homology  and  analogy.  Homologous  structures  have  a  similar 
developmental  origin  and  hereditary  basis,  but  may  or  may  not 
have  a  similar  function.  Analogous  structures,  though  functionally 
similar,  are  otherwise  different.  The  existence  of  many  organs 
diverse  in  function  yet  clearly  similar  in  structure — for  example, 
the  human  hand,  a  seal's  flipper,  and  a  bat's  wing — constitutes  a 
conundrum  best  explained  by  evolution.  The  list  of  morphological 
homologies  can  be  almost  endlessly  extended,  but  the  interpreta- 
tion remains  the  same — namely,  descent  with  modification.  The 
persistence  of  nonfunctional  vestigial  organs  of  all  kinds  is  still 
another  biological  phenomenon  best  accounted  for  by  the  theory 
of  evolution.  The  serial  homologies  demonstrated  in  segmented 
animals  are  indicative  of  the  evolution  of  segmental  diversifica- 
tion from  more  uniformly  segmented  ancestral  stocks.  The  as- 
sumption that  anatomical  homology  and  genetic  relationship  go 
hand  in  hand  has  been  strongly  reinforced  by  the  discovery  of 
homologies  at  the  level  of  the  chromosomes  and  the  genes. 

SUGGESTED  READING 

Davis,  D.  D.,  1949.  "Comparative  anatomy  and  the  evolution  of  the  vertebrates," 

Genetics,  paleontology  and  evolution.  G.  L.  Jepsen,  E.  Mayr,  and  G.  G. 

Simpson,  eds.  Princeton:  Princeton  University  Press. 
Gregory,  W.  K.,  1951.  Evolution  emerging,  2  vols.  New  York:  Macmillan. 
Romer,  A.  S.,  1955.  The  vertebrate  body,  2d  ed.  Philadelphia:  Saunders. 

,  1959.  The  vertebrate  story,  4th  ed.  Chicago:  University  of  Chicago  Press. 

Spencer,  W.  P.,   1949.  "Gene  homologies  and  the  mutants  of  Drosophila  hydei," 

Genetics,  paleontology  and  evolution.  G.  L.  Jepsen,  E.  Mayr,  and  G.  G. 

Simpson,  eds.  Princeton:  Princeton  University  Press. 
Young,  J.  Z.,  1950.  The  life  of  vertebrates.  Oxford:  Clarendon  Press. 


CHAPTER 


u 


Comparative  Biochemistry 


Some  biochemical  traits  are  so  fundamental  that  they  are 
universally  present  in  living  things;  others  are  widespread,  char- 
acterizing large  groups  of  animals  or  plants;  still  other  bio- 
chemical properties  are  species  specific  or  may  even  be  unique  to 
a  given  individual.  Within  this  array  of  similarities  and  differ- 
ences is  to  be  found  considerable  evidence  for  evolution  and  for 
the  solution  of  specific  phylogenetic  problems.  The  term  "homol- 
ogy" is  customarily  associated  with  morphological  characteristics, 
but  biochemical  as  well  as  structural  homologies  can  be  recog- 
nized. Common  ancestry  may  be  indicated  just  as  clearly  by 
homologous  biochemical  compounds  as  by  homologous  morpho- 
logical structures.  This  type  of  evidence,  which  gives  essentially 
an  independent  check  on  the  conclusions  drawn  from  comparative 
studies  in  embryology  and  anatomy,  was  unavailable  to  Darwin. 
Since  biochemical  traits  generally  seem  to  change  more  gradually 
than  morphological  traits,  the  conclusions  drawn  from  biochem- 
ical evidence  are  apt  to  be  more  soundly  based.  In  some  cases, 
biochemical  evidence  has  made  it  possible  to  trace  relationships 
where  previously  no  reliable  conclusions  could  be  drawn  from 
morphology.  As  might  be  expected,  analogous  biochemical  com- 
pounds also  exist;  for  example,  both  hemoglobin  and  hemocyanin 
function  as  oxygen-carrying  respiratory  pigments,  but  they  are 
analagous  rather  than  homologous,  for  hemoglobin  is  an  iron- 
porphyrin  protein  whereas  hemocyanin  is  a  copper  protein. 

Although  different  species  may  differ  radically  in  their 
gross  morphology,  nearly  all  of  them  are  formed  from  similar 


103 


104  •  THE   EVIDENCE    FOR   EVOLUTION 

compounds,  which  are  used  metabolically  in  similar  ways.  An  elm  tree 
and  an  elephant,  a  bacterium  and  a  Bantu  may  at  first  glance  appear 
to  have  little  in  common,  but  at  the  biochemical  level  they  are  much  alike. 
The  hereditary  materials  in  both  plants  and  animals,  for  example,  are  nucleic 
acids,  while  the  stucture  of  the  organism  is  erected  primarily  with  protein  mole- 
cules. The  carbohydrates  and  fats,  on  the  other  hand,  serve  as  the  major  sources 
of  energy  for  carrying  on  metabolic  work.  The  photosynthetic  process  makes 
possible  the  nutritional  independence  of  the  green  plants,  which  are  able  to 
synthesize  organic  compounds  (carbohydrates,  fats,  proteins,  nucleic  acids,  etc.) 
from  simple  substances  such  as  carbon  dioxide,  water,  and  inorganic  salts.  Other 
organisms,  with  few  exceptions,  are  either  directly  or  indirectly  dependent  on 
green  plants  for  their  energy.  Even  for  a  top  carnivore  (which  does  not  serve  as 
prey  to  another  carnivore)  such  as  a  polar  bear,  this  relationship  can  be  traced 
back  through  the  food  chain  to  its  origin  in  the  chlorophyll  of  green  plants. 
Despite  the  diversity  of  form  and  function  found  among  the  different  species  of 
plants  and  animals,  certain  chemical  compounds  play  similar  key  roles  in  their 
metabolism.  In  the  digestion  of  carbohydrates  in  animals,  the  complex  polysac- 
charides are  hydrolyzed  and  broken  down  into  their  constituent  simple  sugars  or 
monosaccharides,  of  which  the  most  important  is  glucose.  The  glucose  molecules, 
after  absorption  from  the  intestine,  become  the  building  blocks  for  the  formation 
of  the  animal's  carbohydrates  such  as  glycogen  or,  by  stepwise  oxidation,  they 
become  the  major  source  of  energy  for  the  variety  of  processes  going  on  within 
the  cells.  Similarly,  proteins  are  broken  down  to  amino  acids,  and  fats  to  fatty 
acids  and  glycerol,  which  then,  after  absorption,  enter  into  the  metabolism  of  the 
animal.  Furthermore,  these  substances  are  to  a  large  extent  interconvertible.  The 
amino  acids,  for  example,  may  undergo  deamination  or  loss  of  the  amino  group, 
which  then  contributes  to  urea  formation.  The  deaminized  portion  may  be  oxi- 
dized, ultimately  to  carbon  dioxide  and  water,  or  it  may  be  synthesized  into 
glucose  or  a  fatty  acid  or  even  into  another  amino  acid.  Thus,  although  the  types 
of  carbohydrates,  fats,  and  proteins  in  different  species  are  distinctive,  many  of 
the  amino  acids,  fatty  acids,  and  simple  sugars  of  which  they  are  composed  are 
identical  in  both  plants  and  animals.  The  metabolic  pathways  they  follow  are 
also  similar.  For  example,  the  ornithine  cycle,  the  Krebs  tricarboxylic  acid  cycle, 
the  cytochrome  system,  the  metabolism  of  aromatic  amino  acids,  glycolysis,  the 
roles  of  actomyosin  and  adenosine  triphosphate  (ATP),  and  many  other  meta- 
bolic sequences  have  been  identified  in  a  wide  variety  of  species.  For  this  reason, 
it  is  possible  to  study  cellular  or  general  physiology,  a  field  that  concentrates  on 
the  phenomena  common  to  the  cells  of  many  different  species.  The  conclusion 
seems  inescapable  that  the  existence  of  these  fundamental  similarities  must  be 
regarded  as  evidence  for  an  underlying  kinship  among  all  living  things.  It  seems 
advisable,  therefore,  to  examine  in  further  detail  the  biochemical  evidence  relat- 
ing to  evolution. 


COMPARATIVE    BIOCHEMISTRY  •   105 


Plant  Pigments 


Some  rather  interesting  information  about  evolution  can  be  derived 
from  a  consideration  of  various  plant  pigments.  Chlorophyll  is  present  in  all 
photosynthetic  organisms,  and  this  biochemical  common  denominator  seems  indic- 
ative of  an  affinity  among  these  species.  Several  types  of  chlorophyll  have  been 
identified,  but  all  have  the  same  basic  porphyrin  or  tetrapyrrole  structure  with 
magnesium  attached  to  the  ends  of  the  pyrroles : 


2n5 


Chlorophyll  a  occurs  in  almost  all  types  of  photosynthetic  organisms,  but  the 
other  kinds  of  chlorophyll  have  a  more  limited  distribution  (see  the  listing  be- 
low) .  Even  the  sulfur  bacteria  contain  chlorophyll-like  compounds. 


group  of  plants 

chlorophylls 

green  plants 

a  and  b 

brown  algae 

<zand  c 

diatoms 

a  and  c 

red  algae 

a  and  d 

yellow-green  algae 

a  and  e 

blue-green  algae 

a 

The  chlorophylls  are  bound  to  proteins  in  the  chloroplasts  and  differ  from  each 
other  only  in  the  side  chains  attached  to  the  outer  ends  of  the  tetrapyrrole 
nucleus.  Descent  with  modification  from  a  common  ancestry  seems  clearly  indi- 
cated for  these  photosynthetic  species. 


106  •  THE    EVIDENCE    FOR   EVOLUTION 

The  anthocyanins  and  anthoxanthins  are  water-soluble  pigments  found 
in  the  cell  sap  of  plants,  and  are  responsible  for  most  of  the  flower  and  fruit 
colors  in  higher  plants  and  for  much  of  the  color  in  autumn  foliage.  The  antho- 
cyanins vary  in  color  from  red  to  purple  to  blue;  the  anthoxanthins,  though 
chemically  quite  similar  to  the  anthocyanins,  appear  yellow  or  white.  The  antho- 
cyanins are  always  combined  with  sugars  to  form  glycosides,  and  the  anthoxan- 
thins are  usually  found  as  glycosides  also.  The  color,  particularly  of  the  antho- 
cyanins, changes  with  the  acidity  of  the  cell  sap,  becoming  bluer  as  the  acidity 
decreases. 


pelargonidin 
(anthocyanin;  pink) 


apigenin 
(anthoxanthin;  ivory) 


(aglycone 
residues) 


The  anthocyanins  and  anthoxanthins  of  many  hundreds  of  species  of 
flowering  plants  have  been  studied  both  genetically  and  biochemically  in  one 
of  the  pioneer  studies  of  biochemical  genetics.  The  results  have  shown  that  these 
pigments  are  apparently  derived  from  a  common  precursor  and  that  the  differ- 
ences among  them  are  due  to  simple  gene  substitutions,  which  determine  the 
state  of  oxidation  and  methoxylation  of  the  side  phenyl  ring,  the  pH  of  the  cell 
sap  of  the  petals,  and  the  position,  number,  and  nature  of  the  attached  sugars. 
Such  similarities,  extending  through  many  families  of  plants,  certainly  seem  a 
strong  argument  for  a  common  origin. 


Photoreceptors 

Even  more  remarkable,  perhaps,  are  the  biochemical  homologies  in- 
volved in  photoreceptor  systems,  both  animal  and  plant.  Phototropism,  photo- 
taxis,  and  vision  are  apparently  all  dependent  on  the  yellow  to  red  fat-soluble 
carotenoid  pigments.  The  carotenes  and  the  related  xanthophylls  are  found  in 
the  chloroplasts,  where  their  color  is  usually  masked  by  the  chlorophyll.  Al- 
though relatively  few  studies  have  been  made  in  plants  or  among  the  lower 
invertebrates,  the  available  evidence  implicates  the  carotenoids  or  their  deriva- 
tives in  the  light  reactions  of  these  groups.  Shown  below  is  /3-carotene,  the  most 
familiar  of  the  carotenoid  pigments. 


COMPARATIVE    BIOCHEMISTRY  •   107 


H     H  CH3   H     H     H   CH3    H     H     H     H   CH3   H      H     H  CH3   H      H 

I       I       I       I       I       I       I       I       I       I       I       I       I       I       I       I       I 

c=c-c=c-c=c-c=c-c=c-c=c-c=c-c=c-c=c 


/^-carotene 

The  taxonomically  intermediate  position  of  the  green  flagellates  such  as 
Euglena,  which  have  been  claimed  as  algae  by  the  botanists  because  they  possess 
chloroplasts  and  as  Protozoa  by  zoologists  because  of  their  other  traits,  is  con- 
firmed by  the  presence  of  the  carotenoid,  astaxanthin,  in  the  eyespot.  Since  this 
group  contains  both  chlorophyll,  a  plant  pigment,  and  astaxanthin,  which  is  an 
exclusively  animal  carotenoid,  it  cannot  properly  be  assigned  to  either  the  plant 
or  the  animal  kingdom. 

The  vertebrates  and  the  higher  invertebrates  such  as  arthropods  and  mol- 
lusks  cannot  synthesize  their  carotenoids  and  must  obtain  them  in  their  nutrition 
as  the  A  vitamins,  ultimately  derived  from  plants.  That  the  A  vitamins  are 
similar  to  the  carotenes  may  be  seen  from  the  structure  of  vitamin  Ax. 


H     H  CH3   H     H     H  CH3   H 

I       I       I       I       I       I       I       I 

C  =  C-C  =  C-C=C-C  =  C-CH20H 


vitamin  Ai 


The  carotenoid  pigments  play  a  fundamental  role  in  photoreception  in 
the  arthropods,  mollusks,  and  chordates.  These  phyla  independently  have  devel- 
oped image-forming  eyes,  each  of  a  distinct  type,  and  yet  each  utilizes  the  A 
vitamins  in  the  photoreception  process.  The  details  have  been  most  carefully 
studied  in  the  vertebrate  eye.  Photoreception  takes  place  in  the  retina,  where  two 
types  of  photoreceptors  are  found:  the  rods,  specialized  for  vision  in  dim  light, 
and  the  cones,  specialized  for  vision  in  bright  light  and  for  color  vision.  The 
action  of  light  on  the  photosensitive  carotenoid-protein  pigments  in  these  cells 
causes  the  carotenoid  to  split  off  from  the  protein,  giving  rise  to  nervous  excita- 
tion, which  is  transmitted  as  a  nervous  impulse  from  the  retina  through  the 
optic  nerve  to  the  brain  where  it  gives  rise  to  visual  sensations.  The  chemistry 


108  •  THE    EVIDENCE    FOR   EVOLUTION 

has  been  most  carefully  worked  out  in  the  rods.  Here  the  photosensitive  pigment 
is  rhodopsin,  a  rose-colored  compound  that  is  broken  down  by  light  through  a 
series  of  steps  to  the  protein,  opsin,  and  to  vitamin  A1  or  its  derivative,  retinenej. 
The  bleached  products  can  regenerate  rhodopsin  spontaneously  in  the  dark. 
Under  continuous  light  the  whole  system  goes  into  a  steady  state  with  the  con- 
tinuous restitution  of  rhodopsin  permitting  vision  to  persist  indefinitely.  The 
phenomenon  of  dark  adaptation,  during  which  the  ability  to  see  in  a  dimly  lit 
room  markedly  increases,  can  readily  be  explained  as  due  to  the  resynthesis  of 
rhodopsin,  which  was  previously  somewhat  depleted  in  the  light.  The  details  of 
the  changes  in  the  rods  are  outlined  in  the  diagram.  (It  may  be  noted  that  the 
rhodopsin  is  formed  only  from  the  so-called  as  optical  configuration  of  retinenej 
but  that  it  breaks  down  to  the  trans  form. ) 


visual  orange 

visual  yellow 

\ 

cis  retinene,  +  opsin  :  — ^tr^nt  retinene,  +  opsin  (protein) 

u  A  ^ZZZI    J  t  A 

as  vitamin  Ax~  trans  vitamin  Ax 

(After  Wald) 

The  rhodopsin  system  utilizing  vitamin  Ax  is  widely  distributed,  being 
found  in  the  retinas  of  marine  and  terrestrial  vertebrates.  The  crustaceans  and 
the  squid,  a  cephalopod  mollusk,  also  use  At  or  retinenej  in  their  visual  pig- 
ments. However,  the  retina  of  fresh-water  fishes  contains  a  different  light- 
sensitive  pigment,  a  purple  substance  known  as  porphyropsin.  The  opsins  are 
essentially  the  same  as  in  rhodopsin,  but  the  carotenoids  are  vitamin  A2  and 
retinene2,  which  differ  from  A1  and  retinenej  in  having  just  one  extra  double 
bond  in  the  ring.  This  finding  poses  some  very  intriguing  questions,  for  there 
are  no  fundamental  phylogenetic  distinctions  between  marine  and  fresh-water 
fishes;  closely  related  species  may  be  found  in  either  environment. 

The  available  evidence  indicates  that  the  ancestral  vertebrates  lived  in 
fresh  water  and  had  porphyropsin  as  their  visual  pigment.  The  evolution  of  the 
vertebrates  gave  rise  to  species  that  invaded  the  oceans  or  the  land,  and  in  both 
cases  the  invasion  of  the  new  habitat  was  accompanied  by  a  shift  from  porphy- 
ropsin to  rhodopsin.  Study  of  the  types  intermediate  in  their  habitats  such  as 
amphibians  or  fishes  migrating  between  the  sea  and  fresh  water  has  shown  that 


COMPARATIVE    BIOCHEMISTRY  •   109 


they  also  are  intermediate  in  their  visual  pigments.  These  findings  are  sum- 
marized below. 


Marine  fishes  (Ai) 

\ 

Catadromous  fishes  (A^Ag) 
(e.g.  eel)   \ 

Anadromous  fishes  (A^A^ 
(e.g.  salmon)    V 

Fresh-water  fishes  t  (A2)' 

Lampreys  (A2) 


Land  vertebrates  (Ax) 
Amphibians  (Ax  and  A2) 


Crustacean  eye 

(Au  retinenei) 


Cephalopod  eye 
(retinenex) 


Invertebrate  phototropisms 
(pigments  unidentified) 


Green  flagellate  orientation 
(astaxanthin) 

Plant  phototropism 
(carotene,  xanthophyll) 


(After  Wald) 


The  type  of  pigment  is  not  simply  an  adaptation  directly  determined 
by  the  environment,  for  one  exceptional  group  of  fish,  the  wrasse  fishes 
(Labridae),  is  exclusively  marine  yet  all  have  porphyropsin.  Furthermore,  the 
sea  lamprey,  which  migrates  from  the  ocean  to  fresh  water  to  spawn,  already  has 
vitamin  A2  and  porphyropsin  as  it  starts  its  migration  from  the  sea.  Thus,  genetic 
control  of  the  type  of  visual  pigment  is  clearly  indicated. 

The  lampreys  are  the  most  primitive  living  vertebrates  and  only  dis- 
tantly related  to  the  fresh-water  bony  fishes  or  teleosts.  Hence,  the  presence  of 
porphyropsin  in  this  group  places  this  type  of  pigment  close  to  the  origin  of  the 
vertebrate  visual  system.  The  lungfish,  which  have  evolved  along  a  separate  line 
of  descent  from  the  modern  fresh- water  teleosts,  also  have  vitamin  A2  in  their 
retinas. 

Among  the  teleosts  the  salmon  and  the  eels  also  migrate  between  the 
sea  and  fresh  water.  Migratory  fish  may  be  divided  into  two  groups:  anadromous, 
which  migrate  from  the  sea  to  fresh  water  to  spawn,  and  catadromous,  which 


110  •  THE    EVIDENCE    FOR   EVOLUTION 

migrate  from  fresh  water  to  spawn  in  the  sea.  The  retinas  of  anadromous  salmon 
contain  both  rhodopsin  and  porphyropsin,  vitamins  Ax  and  A2,  but  the  porphy- 
ropsin  predominates.  The  catadromous  eels  that  return  to  the  sea  to  spawn  also 
have  both  pigments,  with  the  rhodopsin  predominant.  Among  all  of  the  fish  in 
these  groups  thus  far  studied,  it  has  been  found  that  their  visual  pigments  are 
predominantly  or  exclusively  the  kind  ordinarily  associated  with  their  spawning 
environment. 

The  amphibians,  which,  as  their  name  suggests,  live  on  land  or  in  the 
water  or  a  little  bit  of  both,  are  intermediate  between  a  fresh-water  and  a  ter- 
restrial existence.  Their  visual  systems  parallel  their  habitat,  for  those  living  in 
fresh  water,  such  as  tadpoles  or  the  mud-puppy  Necturus,  a  permanently  larval 
aquatic  form,  contain  vitamin  A2,  whereas  terrestrial  forms  such  as  adult  frogs 
have  rhodopsin  and  vitamin  Ax.  Even  within  a  given  species  the  type  of  visual 
pigment  changes  when  metamorphosis  makes  possible  a  change  in  habitat. 

The  vitamin  Aj-retinenej-rhodopsin  system  appears  to  have  originated 
somewhere  in  the  evolutionary  history  of  the  invertebrates,  and  the  vitamin  A2- 
retinene2-porphyropsin  system  appears  to  be  closely  associated  with  the  origin  of 
the  vertebrates.  A  major  unanswered  question  is  why  a  change  from  porphy- 
ropsin to  rhodopsin  should  have  taken  place  when  fresh-water  vertebrates 
evolved  into  marine  or  terrestrial  species.  The  conclusion  that  porphyropsin  con- 
fers an  adaptive  advantage  in  the  fresh-water  environment  and  rhodopsin  is 
better  suited  to  either  an  oceanic  or  terrestrial  existence  seems  inescapable.  The 
change  from  one  system  to  another  within  the  life  cycle  of  a  single  individual 
seems  the  best  indication  that  adaptation  is  involved.  It  must  be  remembered, 
however,  that  these  changes  are  under  genetic  control  and  hence  must  have  been 
brought  about  by  natural  selection  and  not  by  the  direct  influence  of  the  environ- 
ment. 

Immunology 

Some  unusual  and  valuable  information  about  evolution  has  been  de- 
rived from  still  another  type  of  biochemical  study — namely,  immunology.  The 
immunity  of  an  organism  is  based  upon  what  is  called  the  antigen-antibody 
reaction.  An  antigen  is  a  foreign  substance  of  biological  origin  that  is  usually  a 
protein  although  some  polysaccharides  are  also  antigenic.  In  response  to  the 
entrance  of  an  antigen  into  the  body,  an  antibody,  which  is  a  protein  capable  of 
combining  specifically  with  that  antigen,  is  formed.  If  the  antigen  subsequently 
enters  the  body  again,  the  antibodies  already  present  will  combine  with  it,  and 
the  individual  becomes  immune  to  its  harmful  effects.  Antibodies  can  be  devel- 
oped not  only  against  bacteria  and  viruses  but  against  a  variety  of  other  sub- 
stances as  well,  and  this  fact  has  been  utilized  to  study  the  relationships  of 
organisms. 


COMPARATIVE    BIOCHEMISTRY  •   111 

If  the  blood  serum  or  body  fluid  of  an  animal  is  injected  into  a  rabbit, 
the  rabbit  forms  antibodies  in  its  blood  against  the  foreign  serum  proteins.  By 
withdrawing  the  rabbit's  blood  and  removing  the  cells  from  the  serum  it  is  pos- 
sible to  carry  out  the  antigen-antibody  reaction  (foreign  serum-rabbit  antiserum) 
in  a  test  tube,  where  a  precipitate  is  formed.  This  so-called  precipitin  test  or 
various  refinements  of  it  have  been  used  in  a  number  of  phylogenetic  studies,  a 
few  of  which  will  be  mentioned  here. 

Some  of  the  earliest  studies  were  conducted  by  Nuttall.  Perhaps  the 
most  exciting  at  the  time  was  the  discovery  that  rabbit  serum  containing  anti- 
human  antibodies  reacted  almost  as  strongly  with  chimpanzee  serum  as  it  did 
with  human  serum;  somewhat  less  strongly  with  sera  from  the  other  apes;  still 
less  with  monkey  sera;  only  slightly  with  carnivore  and  ungulate  sera;  and  essen- 
tially not  at  all  with  insectivore,  rodent,  and  marsupial  sera.  Because  of  the  spe- 
cificity of  the  antigen-antibody  reaction  these  cross  reactions  are  a  measure  of  the 
degree  of  similarity  of  the  serum  proteins  in  the  different  species.  They  tend  to 
confirm,  therefore,  the  relationships  of  man  to  the  Primates  and  particularly  to 
the  anthropoid  apes. 

In  another  experiment  Nuttall's  group  showed  that  the  horseshoe  crab, 
Limulus,  once  classified  with  the  other  crabs  among  the  Crustacea,  belonged  in- 
stead much  nearer  the  Arachnida,  for  an  anti-Limulus  serum  reacted  strongly 
with  spider  sera,  but  scarcely  at  all  with  crustacean  sera.  A  more  recent  study  by 
Wilhelm  has  shown  a  close  serological  relationship  between  echinoderms  and 
hemichordates,  which  confirms  the  morphological  evidence.  Boyden  has  demon- 
strated that  whales,  which  because  of  their  adaptations  to  marine  life  were  diffi- 
cult to  place  taxonomically  among  the  mammals,  are  most  closely  related  to  the 
cloven-hoofed  Artiodactyls.  Another  study  by  Moody  indicated  that  rabbits  and 
hares,  long  classed  with  the  rodents,  properly  belong  in  the  separate  order 
Lagomorpha  with  closer  affinities,  actually,  to  the  Artiodactyls  than  to  the 
rodents.  Thus,  the  serological  approach  has  been  very  fruitful,  particularly  in 
instances  in  which  the  standard  morphological  methods  were  not  too  reliable. 


►  SUMMARY 


The  field  of  biochemistry  has  developed  since  Darwin's 
time  to  the  point  where  it  now  can  make  notable  contributions  to 
our  knowledge  of  evolution.  Biochemical  as  well  as  structural 
homologies  can  be  recognized,  and  they  furnish  reliable  evidence 
of  relationship  independent  of  the  conclusions  based  on  compara- 
tive morphology.  The  chemical  composition  of  living  organisms, 
based  on  nucleic  acids,  proteins,  carbohydrates,  and  fats,  is  itself 
evidence  for  the  underlying  kinship  of  all  forms  of  life.  Detailed 
studies  of  plant  pigments,  photoreceptor  systems,  immunology, 


112  •  THE    EVIDENCE    FOR    EVOLUTION 

and  many  metabolic  systems  have  led  to  a  variety  of  detailed  bio- 
chemical evidence  on  relationships  within  and  between  groups. 
This  evidence,  unavailable  to  Darwin,  has  confirmed  and  extended 
our  knowledge  of  evolution,  for  no  other  theory  is  adequate  to 
interpret  these  data  or  so  fruitful  in  suggesting  further  research  in 
the  field. 

SUGGESTED  READING 

Boyden,  A.  A.,  1942.  "Systematic  serology:  a  critical  appreciation,"  Physiol.  Zool., 
15:\09-W). 

,  1953.  "Fifty  years  of  systematic  serology,"  Systematic  Serol.,  2:19. 

Florkin,  M.,  1949.  Biochemical  evolution  (S.  Morgulis,  tr.).  New  York:  Academic 
Press. 

Nuttall,  G.  H.  F.,  1904.  Blood  immunity  and  blood  relationship.  New  York:  Cam- 
bridge University  Press. 

Prosser,  C.  L.,  I960.  "Comparative  physiology  in  relation  to  evolutionary  theory," 
Evolution  after  Darwin,  Vol.  I,  The  evolution  of  life.  S.  Tax,  ed.  Chicago: 
University  of  Chicago  Press. 

,  ed.,  1958.  Physiological  adaptation.  Washington,  D.  C:  American  Physio- 
logical Society. 

Wald,  G.,  1952.  Biochemical  evolution.  Modern  trends  in  physiology  and  bio- 
chemistry. New  York:  Academic  Press. 

,  1958.  "The  significance  of  vertebrate  metamorphosis,"  Science,  i28.T481- 

1490. 


CHAPTER 


12 


Biochemical  Adaptation 


Biochemical  as  well  as  morphological  adaptations  can  be 
discerned.  The  morphology  of  the  animal  in  a  sense  simply  re- 
flects its  functioning;  it  is  the  net  result  of  all  of  the  genetic  and 
environmental  influences  acting  upon  the  developing  organism. 
Regulation  of  the  composition  of  the  body  fluids  in  different 
kinds  of  environments  has  led  to  a  variety  of  biochemical  adapta- 
tions. One  of  the  fundamental  similarities  among  living  species 
of  animals  is  in  the  relative  ionic  composition  of  the  body  fluids. 
Although  they  may  differ  in  their  absolute  composition,  neverthe- 
less on  a  relative  basis  the  plasma  of  such  diverse  species  as  the 
jellyfish,  lobster,  frog,  and  man  is  quite  similar,  and  furthermore 
is  much  like  sea  water  (see  Table  12-1).  These  similarities  sug- 
gested to  Macallum  that  the  body  fluids  of  animals  were  originally 
derived  from  sea  water.  Since  it  is  widely  believed  that  life  origi- 
nated in  the  sea,  the  suggestion  seemed  quite  reasonable.  He  even 
accounted  for  the  discrepancies  between  the  concentrations  of 
potassium  and  magnesium  in  human  plasma  and  sea  water  by  the 
fact  that  the  ocean  millions  of  years  ago  contained  less  magnesium 
and  more  potassium  than  at  present.  The  major  difficulty  with  this 
theory  is  that  it  assumes  that  the  body  fluids,  since  being  closed 
off  from  the  sea,  presumably  at  different  times  for  different 
species,  have  somehow  remained  of  the  same  composition  despite 
the  vicissitudes  of  existence  and  evolution  in  the  history  of  each 
species.  Since  the  evidence  is  clear  that  the  ionic  composition  of 
the  body  fluids  is  actively  maintained  by  living  cells,  the  theory  is 
obviously  far  too  simple.  An  alternative  explanation  may  be  that 


113 


114  •  THE    EVIDENCE    FOR   EVOLUTION 

life  can  exist  only  within  rather  narrow  limits  and  arose  at  a  time  when  the  ionic 
composition  of  the  ancient  seas  was  similar  to  that  of  the  plasma  of  present-day 
animals.  These  ionic  limitations  have  remained  essentially  unchanged;  conse- 
quently, all  subsequent  evolution,  no  matter  what  direction  it  took,  of  necessity 
was  accompanied  by  the  development  of  mechanisms  for  maintaining  the  ionic 
composition  of  the  body  fluids  within  the  limits  that  would  support  life.  It  is 
known  that  one  of  the  requirements  for  life  is  enough  water  containing  the 
proper  concentrations  of  the  right  kinds  of  salts. 


TABLE    12-1 

Relative  Ionic  Compositions  of  the  Bloods  and  "Tissue  Fluids  of 

Some  Different  Animals  {After  Macallum  from  Baldwin) 


Na 

K 

Ca 

Mg 

CI 

S03 

Sea  water 

100 

3.61 

3.91 

12.1 

181 

20.9 

King  crab 

Limulus 

100 

5.62 

4.06 

11.2 

187 

13.4 

Jellyfish 

Aurelia 

100 

5.18 

4.13 

11.4 

186 

13.2 

Lobster 

Homarus 

100 

3.73 

4.85 

1.72 

171 

6.7 

Dogfish 

Acanthias 

100 

4.61 

2.71 

2.46 

166 

— 

Sand  shark 

Carcharias 

100 

5.75 

2.98 

2.76 

169 

— 

Cod 

Gadus 

100 

9.50 

3.93 

1.41 

150 

— 

Pollack 

Pollachius 

100 

4.33 

3.10 

1.46 

138 

— 

Frog 

Rana 

100 

— 

3.17 

0.79 

136 

— 

Dog 

Cants 

100 

6.62 

2.8 

0.76 

139 

— 

Man 

Homo 

100 

6.75 

3.10 

0.70 

129 

— 

Aquatic 

Life 

The  maintenance  of  the  proper  concentration  of  salts  is  apparently  a 
relatively  simple  matter  for  most  marine  animals.  A  word  about  osmosis  is 
appropriate  at  this  point.  When  two  different  solutions  are  separated  by  a  semi- 
permeable membrane,  which  permits  passage  of  the  solvent  but  not  of  the  dis- 
solved substances,  the  solvent  will  flow  toward  the  solution  of  higher  concentra- 
tion, thus  tending  to  equalize  the  concentrations.  This  movement  is  known  as 
osmosis  or  the  osmotic  flow,  and  the  pressure  resulting  from  this  flow  is  osmotic 
pressure.  Another  way  to  think  of  osmotic  pressure  is  as  that  amount  of  pressure 
necessary  to  prevent  any  fluid  from  flowing.  A  comparison  of  the  freezing  point 
of  an  aqueous  solution  with  that  of  pure  water  serves  as  a  simple  yet  precise 
indirect  measure  of  the  osmotic  strength  of  that  solution.  In  the  coelenterates, 
echinoderms,  and  mollusks  the  freezing  point  depression  of  the  body  fluids  does 


BIOCHEMICAL   ADAPTATION  •   115 

not  differ  essentially  from  that  of  the  medium  in  which  they  live,  and  therefore 
their  osmotic  problems  are  not  considered  serious.  However,  the  concentration  of 
salts  in  fresh  water  is  very  low,  and  fresh-water  animals  have  mechanisms  for 
regulating  their  osmotic  concentrations  so  that  they  are  osmotically  independent 
of  their  environments.  Various  methods  have  evolved  in  fresh-water  species  for 
osmotic  regulation.  Their  problem,  in  essence,  is  to  get  rid  of  excess  water. 
Semipermeable  boundary  membranes  permit  the  retention  of  salts,  but  water  is 
constantly  seeping  into  the  cells  by  osmosis,  and  must  be  eliminated  in  some 
way  if  the  cells  are  not  to  swell  up  and  burst  due  to  the  osmotic  pressure.  In  the 
fresh-water  protozoans  contractile  vacuoles  constantly  pump  water  out  of  the 
cell.  Some  protozoans  can  eliminate  in  this  fashion  a  volume  of  water  equal  to 
their  own  volume  in  as  little  as  two  minutes.  Species  in  other  groups  may  have 
most  of  the  body  surface  impermeable  to  both  salts  and  water.  The  chitinous 
exoskeleton  of  crustaceans  such  as  the  crayfish,  the  keratin  in  the  integument  of 
various  vertebrates,  and  the  slimy  surface  of  many  fresh-water  species  all  serve, 
to  various  degrees,  to  render  the  body  surface  impermeable.  Excess  water  is  still 
absorbed,  but  is  eliminated  by  the  excretion  of  a  copious  dilute  urine  through 
the  kidneys  of  species  such  as  the  fresh-water  bony  fish  and  frogs.  A  frog,  for 
example,  excretes  on  the  average  one-third  of  its  body  weight  in  water  each  day. 
Man,  with  quite  different  osmotic  problems,  excretes  only  one-fiftieth  of  his 
weight  per  day.  If  the  salt  concentration  is  to  be  kept  higher  than  that  of  the 
environment,  osmotic  work  must  be  done  in  order  to  absorb  salts  against  the 
concentration  gradient.  Fresh-water  fish  have  special  cells  in  the  gills  that  carry 
out  this  function;  mosquito  larvae  absorb  chloride  ions  through  their  anal 
papillae. 

The  marine  teleosts  or  bony  fishes,  in  contrast  to  the  marine  inverte- 
brates, have  an  osmotic  concentration  only  about  one-half  as  great  as  that  of  sea 
water.  Dessication  is  therefore  a  constant  threat,  for  they  tend  to  lose  water  to 
their  environment.  With  the  Ancient  Mariner,  they  can  croak,  "Water,  water, 
everywhere,  nor  any  drop  to  drink."  Although  they  swallow  large  quantities  of 
sea  water,  nevertheless  their  blood  remains  more  dilute  in  salts  than  the  sea 
water  (see  Table  12-2).  The  sea  water  is  absorbed,  salts  and  all,  from  the  in- 
testine, but  the  excess  salt  is  excreted  by  the  so-called  "chloride  secretory  cells" 
in  the  gills.  Thus  in  both  fresh-water  and  marine  bony  fish,  osmotic  regulation  is 
achieved  only  by  the  expenditure  of  energy  to  do  osmotic  work  in  specially 
adapted  cells  in  the  gills.  The  salts  move  in  opposite  directions,  of  course, 
through  the  cells  of  these  two  groups.  Whereas  fresh-water  teleosts  excrete  a 
copious  dilute  or  hypotonic  urine,  marine  teleosts  waste  a  minimum  of  water,  a 
valuable  material  to  them,  in  the  formation  of  urine,  and  their  urine  is  nearly 
isotonic  with  the  blood.  The  numerous  glomeruli  in  the  kidneys  of  fresh-water 
fishes  appear  to  be  adaptations  for  filtering  off  large  amounts  of  water.  Marine 
fishes,  with  the  problem  of  conserving  water,  have  few  glomeruli  and  this  region 


116  •  THE    EVIDENCE    FOR    EVOLUTION 

TABLE    1  2-2 

Freezing  Point  Depression  of  Body  Fluids  in  Animals  (°C) 
(After  Heilbrunn) 


Marine  animals 
Coelcnterata 

Alcyonium  palmatum 
Echinodermata 

Asterias  glacialis 
Annelida 

Sipunculus  nudus 
Mollusca 

Ostrea  edulis 

Octopus  vulgaris 
Arthropoda 

Limulus  polyphemus 

Homarus  americanus 

Maja  verrucosa 
Tunicata 

Ascidia  mentula 
Chondrichthyes 

Mustellus  vulgaris 

Raja  undulata 
Teleostei 

Conger  vulgaris 

Charax  puntacco 

Fresh-water  animals 
Mollusca 

Limnaea  stagnalis 
Annelida 

Hirudo  officinalis 
Crustacea 

Daphnia  magna 

Telphus  fluviatile 
Osteichthyes 

Cyprinus  carpio 

Salmo  jario 

Terrestrial  animals 
Annelida 

Lumbricus  terrestris 
Mollusca 

Helix  aspera 
Insecta 

Decticus  albifrons 

Lymantria  dispar 

Bombyx  mori 
Amphibia 

Rana  esculenta 
Reptilia 

Emys  europea 
Aves 

Chicken  ? 
Mammalia 

Pig 

Horse 

Cat 


Body  fluid 

2.195 

2.295 

2.27-2.31 

2.23 
2.16 

1.90 
1.82 
2.13 

2.08 

2.36 
1.89 

0.77 
1.04 

0.22-0.23 

0.43 

0.20-0.67 
1.17 

0.50 
0.57 

0.45-0.51 

0.37 

0.50 
0.48 
0.73-0.79 

0.40 

0.47 

0.615 

0.615 
0.564 
0.638 


Outer  medium 

2.2 

2.195-2.36 

2.29 

2.11-2.14 
2.11-2.14 

1.82 
1.80 
2.17 

1.98 

2.29 
1.84 

2.14 
2.29 

0.02-0.03 

BIOCHEMICAL   ADAPTATION  •   117 

of  the  kidney  has  the  appearance  of  having  degenerated.  This  difference  in  the 
kidneys  of  marine  and  fresh-water  species  is  also  considered  to  be  evidence  for 
the  fresh-water  origin  of  the  fishes. 

The  marine  elasmobranchs  (sharks,  skates,  and  rays)  have  about  the 
same  amount  of  salts  in  their  blood  as  the  marine  teleosts,  but  they  have  in  addi- 
tion about  2  percent  urea  (ordinarily  a  nitrogenous  waste  product) ,  which  brings 
the  total  osmotic  pressure  to  slightly  higher  than  that  of  sea  water.  The  solution 
of  the  osmotic  problems  posed  by  life  in  the  sea  is  quite  different,  therefore,  in 
teleosts  and  elasmobranchs.  The  urea  is  retained  because  the  gills  are  relatively 
impermeable  to  urea  in  low  concentrations,  and  the  renal  tubule  contains  a 
special  segment  that  reabsorbs  urea  from  the  glomerular  filtrate.  The  shark  and 
its  relatives  resemble  the  fresh-water  teleosts  in  certain  respects,  for  its  kidney  is 
glomerular,  the  osmotic  gradient  tends  to  drive  water  into  the  fish,  and  the  urea- 
absorbing  segment  corresponds  to  the  salt-absorbing  segment  of  the  renal  tubule 
in  fresh-water  bony  fish.  Certain  elasmobranchs  live  in  fresh  waters,  and  it  is 
believed  that  they  are  descended  from  forms  that  at  one  time  lived  in  the  sea 
and  later  invaded  the  rivers.  The  salt  content  in  the  plasma  of  marine  and  fresh- 
water elasmobranchs  is  almost  the  same,  but  the  fresh-water  species  have  only 
about  0.6  percent  urea  rather  than  2  percent.  Since  a  more  copious  dilute  urine 
must  be  produced  than  even  that  of  the  fresh-water  teleosts,  it  would  appear 
advantageous  if  the  urea  content  were  further  reduced  or  even  eliminated  en- 
tirely, but  this  is  apparently  impossible.  During  the  long  period  of  marine  life, 
the  physiology  of  the  elasmobranchs  became  so  completely  adapted  to  the  pres- 
ence of  a  high  concentration  of  urea  that  the  heart  of  fresh-water  elasmobranchs 
will  not  beat  in  its  absence. 

The  presence  of  the  glomerulus,  a  device  for  excreting  water,  is  evi- 
dence to  indicate  that  all  of  the  fishes  originated  in  fresh  water.  Invasion  of  the 
sea  led  to  degeneration  of  glomeruli  in  the  teleosts;  in  the  elasmobranchs,  the 
retention  of  urea  furnished  a  different  means  of  minimizing  water  loss.  See 
Fig.  12-1. 

Terrestrial  Life 

Life  on  land  poses  still  other  biochemical  problems,  for  the  environ- 
ment consists  of  air,  with  an  abundance  of  oxygen  but  a  scarcity  of  water. 
Furthermore,  the  excretion  of  nitrogenous  waste  products  is  more  difficult  in  an 
environment  where  water  is  at  a  premium.  The  problems  involved  in  biochemical 
adaptation  to  terrestrial  life  suggest  that  the  first  land  vertebrates,  the  early 
amphibians,  arose  from  among  the  fresh- water  fishes  rather  than  among  the 
marine  species  living  in  the  littoral  zone.  Two  of  the  major  adaptive  changes 
required  were  the  ability  to  obtain  oxygen  from  the  air  rather  than  from  water 
and  the  ability  to  withstand  dessication.  In  warm,  shallow,  stagnant,  fresh-water 


118  •  THE    EVIDENCE    FOR    EVOLUTION 

pools,  the  oxygen  supply  may  be  virtually  depleted,  and  survival  in  this  habitat 
may  depend  on  the  ability  of  the  species  to  obtain  the  necessary  oxygen  from  air 
rather  than  water.  The  air  sac  in  fish  is  used  as  a  lung  by  many  species,  particu- 
larly those  dwelling  in  stagnant  waters  or  in  areas  with  seasonal  droughts.  The 
Dipnoi  or  lungfishes  are  perhaps  the  most  familiar  group  of  this  kind,  but  the 
more  primitive  ray-finned  fishes  (Actinopterygii)  such  as  the  spoon-billed  cat 
(Polyod  on-Chondrostei)  and  the  gar  pike  and  bowfin  (Lepisosteus  and  Amia- 


< 

cO 

LU 

CO 

1— 

UJ 

< 

UJ 

CO 

co 

> 

z 

UJ 

1— 

8 

CO 

1— 
co 

UJ 

> 

UJ 

—i 

U 

Z 

Z 

UJ 

1— 

i 

LU 

Qi 

\— 

< 

LU 

3 

<      2 

co  < 

£  <"! 

<  co^t 


" 


m 


i 


iiifiij 


^ 


SEA  WATER 


n 


1  w 


RIVER  WATER 


Fig.  12-1.     Osmotic  pressures  of  bloods  of  various  animals  compared 

with  those  of  fresh  and  sea  waters.         A  =  freezing  point  depression. 

(After  Baldwin.) 


Holostei)  also  use  the  air  sac  as  a  lung  for  getting  oxygen  from  the  air.  The  use 
of  the  air  sac  as  a  swim  bladder  or  hydrostatic  organ  in  the  teleosts  appears  to 
have  been  a  subsequent  development  in  marine  fishes.  The  modern  lungfish 
Protopterus,  during  the  seasonal  drought  in  its  habitat  in  Africa,  estivates  in  a 
slimy  cocoon,  breathing  by  means  of  its  lungs  so  that  it  is  able  to  withstand 
dessication  and  obtain  oxygen  from  air,  the  two  requirements  mentioned  above. 
Furthermore,  the  fresh-water  fish  typically  have  an  integument  of  low  surface 
permeability  to  water,  although  water  enters  quite  freely  through  the  gill  and 
oral  membranes.  Thus,  in  making  the  transition  from  fresh  water  to  land,  the 


BIOCHEMICAL   ADAPTATION  •   119 

problem  is  to  control  water  loss  at  these  points  rather  than  over  the  entire  body 
surface. 

It  is  doubtful  that  marine  fishes  were  the  first  vertebrates  to  invade  the 
land,  since  the  littoral  zone  is  a  rather  stable  environment  with  an  abundant 
oxygen  supply  and  is  therefore  unlikely  to  require  the  major  adaptive  shifts  that 
accompanied  the  origin  of  terrestrial  vertebrates.  Some  of  the  most  slowly  evolv- 
ing groups,  such  as  the  oysters  (Mollusca)  and  the  horseshoe  crab  (Arthro- 
poda),  inhabit  the  littoral  zone,  and  their  slow  rate  of  evolution  can  probably 
be  attributed  to  the  stability  of  their  environment  and  hence  to  the  absence  of 
major  shifts  in  the  pressures  of  natural  selection  that  would  be  expected  to 
produce  rapid  evolutionary  change. 

Among  terrestrial  vertebrates  water  conservation  is  a  major  problem. 
In  most  of  the  amphibians,  evaporation  from  the  body  surface  occurs  at  a  fairly 
rapid  rate  even  though  the  skin  is  not  completely  permeable  to  the  outward  flow 
of  water.  No  amphibian  is  altogether  independent  of  a  moist  environment,  for 
even  the  desert  toads  tend  to  burrow  and  seek  out  damp  and  humid  places.  The 
integuments  of  the  reptiles,  birds,  and  mammals  are  far  more  effective  protection 
against  surface  evaporation,  for  their  permeability  to  water  is  extremely  low.  The 
arthropods,  the  other  major  group  of  animals  to  have  achieved  virtually  complete 
independence  from  a  moist  environment,  are  protected  against  surface  evapora- 
tion by  the  chitinous  exoskeleton.  Both  chitin  and  the  cuticular  wax  contribute  to 
the  impermeability  of  the  cuticle. 

Water  loss  during  excretion  is  minimized  in  terrestrial  forms  in  various 
ways.  The  ancestral  vertebrates  were  fresh-water  fishes  whose  kidneys  primarily 
functioned,  by  means  of  large  glomeruli,  to  rid  the  body  of  excess  water.  The 
frog  kidney  still  functions  in  this  fashion.  In  living  reptiles,  water  loss  has  been 
reduced  through  a  decrease  in  the  size  of  the  renal  corpuscles,  and  consequently 
a  smaller  volume  of  filtrate  is  produced.  In  the  snakes  and  lizards,  the  urine  may 
even  be  solid  or  semisolid.  The  birds  and  mammals  have  renal  corpuscles  of 
normal  size  and  therefore  produce  a  large  volume  of  filtrate,  but  the  kidney 
tubule  is  modified  by  the  presence  of  the  long,  thin  loop  of  Henle  in  which  it  is 
thought  most  of  the  water  resorption  occurs.  Some  water  is  reabsorbed  in  any 
type  of  kidney  tubule,  but  in  man,  for  instance,  with  a  long  kidney  tubule  includ- 
ing the  loop  of  Henle,  scarcely  1  percent  of  the  filtrate  from  the  glomeruli  ever 
reaches  the  bladder.  The  urine  therefore  is  hypertonic  to  the  blood  in  the  birds 
and  mammals.  In  birds,  further  water  absorption  occurs  in  the  cloaca,  and  thus 
the  urine  becomes  a  semisolid  mass.  Insects,  too,  conserve  water  by  reabsorption 
from  the  excretory  wastes,  which  are  discharged  from  the  Malpighian  tubules 
into  the  hind  gut  where  resorption  occurs. 

Terrestrial  animals  obtain  water  by  drinking,  or  with  their  food,  or  as 
a  product  of  metabolism.  Absorption  of  water  occurs  in  the  small  and  large  in- 


120  •  THE    EVIDENCE    FOR    EVOLUTION 

testine,  and  so  the  feces  are  usually  semisolid  or  solid.  The  oxidation  of  organic 
compounds  is  a  major  source  of  water  for  some  species,  particularly  desert 
species  or  such  insects  as  clothes  moths.  The  figures  below  indicate  the  efficiency 
of  formation  of  metabolic  water: 

Oxidation  of 

100  g  of  G  of  water 
protein  41.3 

carbohydrate  55.5 

fat  107.1 

Thus  the  fats,  which  are  frequently  stored  by  desert  mammals,  produce  almost 
twice  as  much  metabolic  water  per  gram  oxidized  as  the  other  compounds. 

Development  of  amniote  embryos  on  land  is  possible  despite  the  fact 
that  they  are  essentially  aquatic.  A  watery  environment  is  provided  for  reptilian 
and  avian  embryos  by  the  shelled  egg  and  for  mammalian  embryos  by  the  uterus 
of  the  mother.  Among  the  amphibians  the  majority  of  species  lay  their  eggs  in 
the  water,  and  an  aquatic  larva,  the  tadpole,  lives  there  for  a  considerable  period. 
However,  a  variety  of  adaptations  exist  in  various  species  of  Amphibia  for  get- 
ting the  eggs  out  of  the  water  and  minimizing  the  larval  period.  The  reptilian 
egg  may  be  regarded  as  the  most  successful  of  these  adaptations.  Viviparity  is  a 
further  modification  of  reptilian  development  that  has  appeared  not  only  in  the 
mammals  but  also  independently  in  certain  reptilian  groups  as  well. 

Nitrogen  Excretion 

Nitrogenous  wastes  from  protein  metabolism  are  excreted  in  a  variety 
of  forms,  with  the  type  of  waste  product  clearly  related  to  the  availability  of 
water  in  the  environment  of  the  organism.  Species  with  an  abundant  water 
supply  excrete  nitrogen  primarily  in  the  form  of  ammonia,  a  soluble  but  highly 
toxic  compound.  Although  no  group  excretes  just  one  nitrogenous  waste  product, 
the  aquatic  invertebrates  and  the  fresh-water  teleosts  primarily  eliminate  am- 
monia, much  of  it  through  the  gills  in  these  teleosts  rather  than  the  kidneys. 
Marine  teleosts,  with  quite  a  different  osmotic  problem  as  described  earlier, 
excrete  considerable  ammonia,  but  they  also  excrete  some  urea  and  up  to  a  third 
of  their  nitrogen  as  trimethylamine  oxide,  the  latter  two  substances  being  soluble 
and  relatively  nontoxic.  The  elasmobranch  fishes,  which  retain  up  to  2.5  percent 
urea  in  the  blood,  also  excrete  it  from  the  gills.  Terrestrial  animals  primarily 
excrete  urea  or  else  uric  acid,  which  has  a  low  toxicity  and  is  relatively  quite 
insoluble,  hence  can  either  be  stored  or  eliminated  as  crystals. 

In  frogs,  the  tadpoles  eliminate  40  percent  or  more  of  their  nitrogen  as 
ammonia,  but  adult  frogs,  with  a  greater  need  for  conservation  of  water,  excrete 
less  ammonia  and  about  80  percent  urea.  Salts  and  some  water  are  reabsorbed  in 


BIOCHEMICAL    ADAPTATION  -121 
NH3  CH3 

ammonia  j  NH2 — C — NH 

CH3— N— CH3  || 

II  O 

O  urea 

trimethylamine 
oxide 

H— N— 0=0 

I       I 
0=C     C— NH 

I      II 
H— N— C— NH 
uric  acid 


O 


the  kidney  tubules,  and  the  evidence  indicates  that  urea  is  actively  secreted  into 
the  tubules.  Mammals  also  excrete  urea,  during  both  embryonic  and  adult  stages, 
and  the  urea  may  be  concentrated  up  to  100  times  its  level  in  the  blood  by  the 
reabsorption  of  water  in  the  kidney  tubules. 

Insects,  birds,  snakes,  and  lizards  eliminate  a  semisolid  urine  containing 
uric  acid  crystals,  thus  minimizing  water  loss  more  than  any  other  group.  It 
should  be  noted  that  in  these  species  with  eggs  protected  against  water  loss 
(cleidoic  eggs)  the  insoluble,  nontoxic  uric  acid  crystals  can  be  stored  in  the 
allantois  during  the  development  of  the  embryo. 

Metamorphosis  from  a  tadpole  to  a  frog  involves  a  number  of  dramatic 
morphological  changes  taking  place  in  a  relatively  short  time.  As  a  result  the 
organism  changes  from  an  aquatic  gill-breathing  herbivore  to  a  terrestrial  lung- 
breathing  carnivorous  tetrapod.  Just  as  striking  as  the  changes  in  structure  are 
the  biochemical  changes  that  accompany  metamorphosis.  At  that  time  nitrogen 
excretion  shifts  over  primarily  to  urea  from  amomnia,  the  visual  pigment  changes 
from  porphyropsin  to  rhodopsin,  and  the  hemoglobin  changes  to  a  type  with  a 
decreased  affinity  for  oxygen.  It  also  has  a  declining  affinity  for  oxygen  as  the 
acidity  increases,  the  so-called  Bohr  effect.  Tadpole  hemoglobin  exhibits  no  Bohr 
effect  and  has  a  relatively  high  affinity  for  oxygen.  These  three  changes  can  be 
regarded  as  adaptive  for  terrestrial  life  although  the  evidence  that  this  is  so  for 
rhodopsin  is  not  yet  available.  They  may  also  be  considered  as  instances  of  bio- 
chemical recapitulation.  The  ancestors  of  the  amphibians  were  fresh-water  fishes, 
which  excreted  primarily  ammonia,  had  porphyropsin  in  their  retinas,  and  pos- 
sessed hemoglobin  of  high  oxygen  affinity  and  a  small  Bohr  effect.  It  is  difficult 
to  avoid  the  conclusion  that  the  developing  frog  manifests  not  only  morpho- 
logical but  biochemical  recapitulation  of  a  phylogenetic  sequence. 

From  this  brief  review,  it  seems  clear  that  the  biochemical  approach  to 
evolutionary  problems  and,  conversely,  the  evolutionary  approach  to  biochemical 
problems,  are  promising  fields  for  further  work,  for  this  is  an  area  of  research 
where  the  surface  has  only  been  scratched. 


122  •  THE    EVIDENCE    FOR   EVOLUTION 

SUMMARY  < 


The  adaptations  of  living  organisms  to  their  environ- 
ments are  biochemical  in  addition  to  being  morphological  and 
behavioral.  Despite  the  varied  osmotic  problems  posed  by  the  sea, 
fresh  water,  and  the  land,  living  things  must  maintain  the  ionic 
composition  of  their  body  fluids  within  rather  narrow  limits. 
Water  intake,  water  conservation,  and  the  excretion  of  metabolic 
waste  products  are  interrelated  problems,  the  solutions  of  which 
vary  greatly  depending  upon  the  environment.  The  invasion  of 
fresh-water  and  terrestrial  habitats  became  possible  only  when 
species  had  evolved  methods  of  osmotic  regulation  in  these  new 
habitats.  Evolutionary  theories,  therefore,  must  account  for  the 
origin  of  biochemical  adaptation  as  well  as  the  somewhat  more 
obvious  morphological  adaptations. 

SUGGESTED  READING 

Baldwin,  E.,  1949.  Comparative  biochemistry,  3d  ed.  New  York:  Cambridge  Uni- 
versity Press. 

Prosser,  C.  L.,  and  F.  A.  Brown,  Jr.,  1961.  Comparative  animal  physiology,  2d  ed. 
Philadelphia:  Saunders. 

Smith,  H.  W.,  1953.  From  fish  to  philosopher.  Boston:  Little,  Brown. 


CHAPTER 


13 

Evolution  in  Animals 


Approximately  a  million  species  of  animals  have  been 
described;  in  some  groups  such  as  birds  and  mammals  virtually  all 
species  are  known,  but  in  others  many  more  species  undoubtedly 
remain  to  be  discovered.  The  great  number  of  living  species  prob- 
ably represents  less  than  1  percent  of  all  of  the  species  that  have 
ever  existed.  These  species  have  been  arranged  into  a  relatively 
small  number  of  phyla,  although  there  is  no  universal  agreement 
among  zoologists  as  to  just  how  many  phyla  there  are.  The  com- 
mon practice  of  arranging  the  different  groups  into  a  phylogenetic 
sequence  is  frequently  a  useful  teaching  device.  The  record  is 
spotty,  however,  and  its  better  known  parts  consist  largely  of 
modern  species  out  at  the  tips  of  the  evolutionary  branches.  Since 
the  phylogenetically  significant  portions  of  the  record  may  be 
obscured  far  in  the  distant  past,  too  great  stress  on  the  phylo- 
genetic arrangement  of  known  groups  may  confuse  the  student 
rather  than  convince  him  of  the  validity  of  the  postulated  rela- 
tionships. 

One  of  the  problems  in  the  discussion  of  evolution  in 
the  animal  kingdom  is  the  lack  of  familiarity  of  many  people  with 
the  major  groups  of  animals.  This  need  not  be  an  insurmountable 
obstacle.  Most  Americans  can  recognize  at  sight  not  only  the 
make  but  the  model  and  year  of  any  car  they  spot  on  the  highway. 
The  number  of  phyla  of  animals  is  roughly  comparable  to  the 
number  of  makes  of  American  automobiles,  and  it  should  be  no 
more  difficult  to  learn  to  distinguish  the  phyla  than  it  is  to  iden- 
tify cars.   Furthermore,  to  remain  unfamiliar  with  at  least  the 


123 


124  •  THE    EVIDENCE    FOR   EVOLUTION 

major  animal  groups  is  to  be  painfully  ignorant  of  the  world  in  which  we  live. 
Therefore,  with  no  further  apologies,  we  shall  consider  the  major  groups  of 
animals  and  the  ways  in  which  they  are  thought  to  be  related  to  one  another. 
Obviously,  many  details  must  be  omitted  in  our  discussion,  and  if  further  in- 
formation about  any  of  the  groups  is  desired,  the  references  at  the  end  of  this 
chapter  should  be  consulted. 

A  word  or  two  may  be  in  order  about  the  nature  of  an  animal.  Anyone 
can  tell  the  difference  between  a  tree,  which  we  call  a  plant,  and  a  cow,  which 
is  an  animal.  The  tree  stands  still  and  ignores  you;  the  cow  moves  about,  appears 
to  see  you,  and  may  even,  if  so  inclined,  kick  or  toss  or  bite  you.  The  tree  makes 
its  own  food  by  photosynthesis  from  simple  inorganic  substances,  but  the  cow 
cannot.  However,  not  all  animals  can  move,  and  not  all  plants  are  sessile,  and 
distinctions  based  on  behavior  and  nutrition  soon  begin  to  weaken.  They  break 
down  completely  in  the  flagellates  or  Mastigophora,  which  have  traits  regarded 
as  characteristic  of  both  animals  and  plants.  The  free-living  flagellate,  Euglena, 
is  in  many  respects  like  an  animal  yet  it  contains  chlorophyll  and  can  therefore 
synthesize  its  own  food.  On  the  other  hand,  it  can  also  absorb  nutrients  from  its 
environment.  It  is  not  surprising  that  both  botanists  and  zoologists  have  laid 
claim  to  such  species,  the  botanists  classifying  them  among  the  algae,  the  zoolo- 
gists among  the  Protozoa.  The  truth  of  the  matter  is  that  there  is  no  sharp  line 
of  demarcation  by  which  animals  may  be  separated  from  plants.  The  living 
world  is  not  divided  into  two  camps,  one  plant,  the  other  animal;  rather,  it 
forms  a  continuum.  It  is  generally  thought  that  the  other  Protozoa  and  the  higher 
multicellular  animals  or  Metazoa  as  well  as  the  higher  plants  have  arisen  from 
ancestral  primitive  flagellates. 

Protozoa 

The  Protozoa  are  fundamentally  single-celled  animals.  Although  some 
form  colonies,  nevertheless  each  cell  is  typically  morphologically  and  physio- 
logically independent.  (The  Protozoa  have  also  been  called  acellular  animals 
because  the  high  degree  of  complexity  in  some  Protozoa  outstrips  anything  to  be 
seen  in  any  individual  metazoan  cell.  However,  since  the  Metazoa  seem  to  have 
been  derived  from  the  Protozoa,  metazoan  cells  may  perhaps  best  be  thought  of 
as  having  lost  some  of  the  versatility  of  the  ancestral  protozoan  cell  in  their  evo- 
lution to  their  present  well-differentiated  and  specialized  functions.  The  Protozoa 
do  have  a  nucleus,  cytoplasm,  a  plasma  membrane,  and  the  other  structures  usu- 
ally associated  with  cells;  hence  by  the  usual  criteria  it  is  difficult  to  avoid  the 
conclusion  that  they  are  cells,  highly  versatile  cells,  but  cells  nevertheless.)  The 
classification  of  the  Protozoa  into  five  classes  based  primarily  on  their  mode  of 
locomotion  is  as  follows: 
1.  Flagellata  (Mastigophora) — propelled  by  one  or  several  flagella.  (A  flagel- 

lum  is  a  long  whiplike  cell  process,  often   regarded  as  a  very  long 

mobile  cilium.) 


EVOLUTION    IN    ANIMALS  •   125 

2.  Sarcodina    (Rhizopoda) — amoeboid   movement   by   means   of   pseudopodia 

(temporary  protrusions  of  the  protoplasm) . 

3.  Sporozoa — all  are  internal  parasites  without  locomotor  organelles,  usually 

producing  spores. 

4.  Ciliata — move  by  means  of  numerous  cilia  (short  hairlike  cell  processes  capa- 

ble of  vibratory  movement) . 

5.  Suctoria — ciliated  only  in  the  young  stages;  as  adults,  have  one  or  more 

suctorial  tentacles. 

The  relationships  among  the  Protozoa  are  by  no  means  clear,  and  their 
classification  is  to  some  extent  quite  arbitrary.  Some  of  the  green  flagellates  can 
hardly  be  separated  from  the  green  algae,  and  other  flagellates,  known  as  the 
chrysomonads,  are  continuous  with  the  filamentous  brown  algae  (Chrysophy- 
ceae).  The  chrysomonads  show  affinities  in  several  directions;  they  may  lose  their 
flagella  and  resemble  algae,  or  lose  their  chromoplasts  and  resemble  animallike 
protomonads,  or  by  the  loss  of  both  flagella  and  chromoplasts  come  to  resemble 
typical  amoebae  or  rhizopods.  Loss  of  the  chloroplasts  in  the  different  orders  of 
flagellates  has  apparently  given  rise  to  the  colorless  animal  forms.  Furthermore, 
some  parasitic  flagellates  with  sporulation  as  a  means  of  reproduction  suggest  the 
affinities  of  this  group  with  the  Sporozoa.  The  relationship  between  the  flagel- 
lates and  the  Sarcodina  is  also  suggested  by  the  Rhizomastigina,  which  typically 
have  both  flagella  and  pseudopodia,  as  well  as  by  the  sporadic  occurrence  of 
amoeboid  forms  among  various  groups  of  flagellates.  That  the  Sarcodina  are 
derived  from  the  flagellates  rather  than  vice  versa  is  suggested  by  the  fact  that 
they  very  often  have  flagellate  immature  stages,  while  the  flagellates  do  not  have 
amoeboid  young  stages. 

The  flagellates  may  very  well  be  a  polyphyletic  group — that  is,  derived 
from  a  number  of  different  sources,  in  this  instance,  spirochaetes  and  bacteria, 
which  in  many  cases  also  have  flagella.  The  rhizopods,  like  the  flagellates,  also 
appear  to  have  a  polyphyletic  origin  from  several  different  groups  of  flagellates. 
The  origins  of  the  Sporozoa  are  again  somewhat  of  an  enigma;  possibly  they  are 
polyphyletic  also.  The  ciliates  and  the  suctorians  are  probably  related,  but  their 
relations  to  the  other  protozoa  are  unclear  although  it  has  been  suggested  that 
the  cilia  are  derived  from  flagella. 

Porifera 

The  enormous  diversity  of  form  and  function  among  the  Protozoa, 
from  the  simplest  amoeba  to  the  most  complex  ciliate,  is  so  great  that  the 
Protozoa  are  sometimes  regarded  as  a  subkingdom,  separate  from  all  of  the 
multicellular  animals  or  Metazoa.  Among  the  multicellular  animals  the  sponges 
or  Porifera  (pore  bearers)  are  regarded  as  an  evolutionary  dead  end  from  which 
no  other  groups  have  evolved.  Therefore,  they  have  been  placed  in  a  separate 


Deuterostomia 


Protostomia 
A. 


PORIFERA 


ENTOPROCTA 


?  MESOZOA 


COELENTERATA 


Fig.  13-1.     (facing  and  above).    The  phylogeny  of  the  animal  kingdom. 


128  •  THE    EVIDENCE    FOR   EVOLUTION 

branch  of  the  Metazoa  called  the  Parazoa.  The  sponges  are  rather  simple  sessile 
organisms,  either  asymmetrical  or  with  radial  symmetry.  They  have  a  cellular 
grade  of  construction  with  special  cells  for  special  functions.  There  are  no 
organs,  no  mouth,  and  no  nervous  tissue.  The  body  is  permeated  with  pores  and 
canals  through  which  water  currents  flow.  The  currents  are  generated  by  the 
flagella  of  the  collar  cells  or  choanocytes  that  line  the  canals  or  chambers.  Food 
particles  are  trapped  by  the  collar  cells  and  are  digested  intracellularly.  There  is 
an  internal  skeleton  of  spicules  or  of  spongin  fibers,  which  the  Greeks  used  to 
line  their  helmets  and  which  we  use  today  to  wash  windows  or  automobiles.  Be- 
cause of  their  characteristic  choanocytes  the  sponges  have  been  considered  de- 
scended from  the  group  of  flagellates  known  as  the  choanoflagellates.  However, 
it  is  also  true  that  sponge  larvae  have  typical  flagellate  cells  rather  than  choano- 
cytes and  hence  the  Porifera  could  have  originated  from  some  more  generalized 
flagellate  stock.  The  sponges  have  not  evolved  too  far  beyond  the  stage  reached 
by  colonial  flagellates;  although  the  cells  are  somewhat  differentiated  and  spe- 
cialized for  particular  functions,  coordinated  activity  has  not  been  possible  be- 
cause of  the  absence  of  any  sort  of  a  nervous  system.  Evolution  in  the  sponges 
has  led  to  increased  complexity  in  the  skeleton  and  in  the  system  of  water  canals 
but  not  to  any  higher  or  more  complex  organisms. 

Mesozoa 

The  phylogenetic  position  of  the  Mesozoa  is  not  at  all  clear.  One  rea- 
son for  this  difficulty  is  th?t  all  of  the  species  in  the  group  are  invertebrate 
parasites,  and  it  cannot  be  said  with  certainty  whether  their  simple  structure  is 
truly  primitive  or  the  result  of  the  degenerative  changes  so  frequent  in  parasites. 
The  Mesozoa  are  small  wormlike  animals  of  extremely  simple  two-layered  solid 
construction.  Whereas  the  inner  layer  of  the  Metazoa  is  digestive  in  function,  in 
the  Mesozoa  it  consists  of  only  one  or  a  few  reproductive  cells.  The  outer  layer 
of  ciliated  cells  carries  on  intracellular  digestion.  This  type  of  structure  shows 
some  resemblance  to  the  ciliated  planula  larva  of  the  coelenterates,  and  the 
Mesozoa  have  sometimes  been  treated  with  this  group.  In  other  cases  they  have 
been  considered  as  degenerate  flatworms.  In  view  of  the  doubts  about  their 
origin  and  affinities  it  seems  best  to  put  them  in  a  separate  branch  of  the 
Metazoa.  Until  more  evidence  is  available,  however,  it  seems  unwise  to  place  too 
great  emphasis  on  their  phylogenetic  importance  as  possibly  the  most  primitive 
group  of  Metazoa. 

Coelenterata 

The  Coelenterata  (coel-enteron  =  hollow  gut),  which  include  such 
forms  as  corals,  jellyfish,  and  sea  anemones,  have  a  gastrovascular  or  digestive 
cavity  with  a  mouth  but  no  anus,  whence  their  name.  They  are  tentacle-bearing, 


EVOLUTION    IN    ANIMALS  •   129 

radially  symmetrical  Metazoa  with  a  tissue  level  of  construction.  Their  cells,  un- 
like the  Porifera,  are  organized  into  an  outer  protective  epithelium  or  ectoderm 
and  an  inner  digestive  layer  or  endoderm.  Though  commonly  called  diploblastic 
(having  two  tissue  layers),  the  coelenterates  also  have,  to  varying  degrees,  indi- 
cations in  the  mesogloea  of  a  third  intermediate  mesodermal  layer.  Their  activ- 
ities are  coordinated  by  a  nerve  net  so  tHat  food  can  be  seized  by  the  tentacles 
and  brought  to  the  mouth.  Whether  in  the  form  of  a  sessile  cylindrical  polyp  or 
a  free-floating  bell-shaped  medusa  or  jellyfish,  the  tentacles  typically  bear  sting- 
ing cells  or  nematocysts. 

Ctenophora 

The  Ctenophora,  the  comb  jellies  or  sea  walnuts,  are  a  small  group  of 
about  80  marine  species;  although  frequently  included  in  the  Coelenterata,  they 
are  sufficiently  distinct  to  warrant  being  placed  in  a  separate  phylum.  They 
take  their  name,  comb-bearing,  from  eight  rows  of  ciliary  combs  used  for  loco- 
motion. Tentacles  are  present  in  most  species,  but  nematocysts,  so  typical  of 
coelenterates,  are  completely  absent.  Symmetry  is  biradial,  a  combination  of 
radial  and  bilateral  traits.  They  resemble  the  coelenterates  in  having  a  gastro- 
vascular  cavity  and  in  having  essentially  a  tissue  level  of  construction,  but  the 
presence  of  mesenchymal  muscle  fibers  in  the  abundant  mesogloea  and  of  an 
aboral  sensory  region  suggests  a  higher  level  of  organization  than  that  of  the 
coelenterates. 

Platyhelminthes 

In  the  flatworms  or  Platyhelminthes,  still  greater  complexity  of  organ- 
ization can  be  observed.  The  flatworms  are  bilaterally  symmetrical;  that  is,  they 
have  anterior  and  posterior  ends,  dorsal  and  ventral  surfaces,  and  right  and  left 
sides,  one  the  mirror  image  of  the  other.  Here  there  are  clearly  three  germ  layers 
with  the  mesoderm  between  the  ectoderm  and  endoderm  giving  rise  to  muscles 
and  other  structures  permitting  greater  complexity  and  efficiency.  The  flatworms 
have  an  organ  level  of  construction,  for  their  tissues  are  associated  to  form 
various  organs.  The  excretory  system  is  of  the  protonephridial  type,  consisting  of 
terminal  flame  bulbs  leading  into  excretory  ducts.  The  flame  bulbs  lie  in  the 
body  fluid  and  wastes  diffuse  across  them  into  the  ducts  where  a  ciliary  tuft  (the 
"flame")  presumably  sets  up  a  current  in  the  duct.  The  nervous  system  has  a 
pair  of  enlarged  anterior  ganglia  and  one  to  three  pairs  of  longitudinal  nerve 
cords.  Hence,  it  is  a  central  nervous  system  rather  than  a  nerve  net.  Like  the 
coelenterates,  most  of  the  flatworms  have  a  gastrovascular  cavity  with  a  single 
opening  that  serves  both  as  a  mouth  and  anus.  They  completely  lack  any  sort  of 
body  cavity  comparable  to  the  coelom  of  higher  forms.  Included  in  the  Platy- 
helminthes are  three  quite  distinct  classes,  the  free-living  flatworms  such  as 


130  •  THE    EVIDENCE    FOR    EVOLUTION 

Planaria  of  the  class  Turbellaria,  the  parasitic  flukes  or  Trematoda,  and  the  in- 
testinal parasites  of  vertebrates,  the  tapeworms  or  Cestoda.  Associated  with  the 
parasitic  habit,  the  parasitic  flukes  and  tapeworms  show  varying  degrees  of 
change  from  the  free-living  turbellarians. 

Origin  of  the  Metazoa 

While  there  is  fairly  wide  agreement  that  the  Porifera  are  derived  from 
the  choanoflagellates,  the  origin  of  the  other  Metazoa  has  been  a  moot  question. 
A  variety  of  possibilities  has  been  raised,  but  no  one  theory  can  be  said  to  have 
a  preponderance  of  evidence  in  its  favor.  However,  in  a  negative  sense  it  is 
possible  by  a  brief  review  of  these  theories  to  see  which  phyla  are  not  likely  to 
have  been  involved,  and  thus  narrow  the  field  considerably.  The  Metazoa  sim- 
plest in  structure  are  the  Porifera,  Mesozoa,  Coelenterata,  Ctenophora,  and  the 
Platyhelminthes.  We  have  already  considered  and  more  or  less  discarded  the 
Porifera  and  Mesozoa,  which  leaves  the  other  three  phyla.  Of  these,  the  co- 
elenterates  and  the  flatworms  are  the  two  groups  most  commonly  considered  as 
lying  closest  to  the  original  Metazoa.  It  should  be  realized  that  the  fossil  record 
has  been  of  no  help  in  settling  the  question  of  the  origin  of  the  Metazoa,  for 
the  presence  of  most  of  the  major  phyla  among  the  fossils  of  the  early  Paleozoic, 
when  the  record  first  becomes  fairly  good,  indicates  that  the  Metazoa  must  have 
arisen  well  back  in  the  Pre-Cambrian.  Therefore,  the  various  theories  are  pri- 
marily speculative  and  all  could  very  well  be  wrong.  The  following  theories  are 
among  the  more  prominent  concepts  thus  far  advanced. 

1.  The  gastraea  theory  of  Haeckel  may  be  regarded  as  the  classical 
theory  of  metazoan  origin,  certainly  it  is  the  most  widely  quoted.  In  its  current 
form,  colonial  flagellates  similar  to  Volvox,  which  forms  a  hollow,  spherical 
colony,  are  equated  with  the  hollow  spherical  blastula  stage  in  the  embryology 
of  the  Metazoa.  This  hypothetical  organism,  termed  the  blastaea,  was  supposed 
to  have  a  single  layer  of  flagellated  cells  and  to  swim  about  with  one  end  always 
forward  so  that  an  antero-posterior  axis  was  established.  The  first  differentiation 
was  assumed  to  be  into  somatic  or  body  cells  and  reproductive  cells,  a  phe- 
nomenon also  observed  in  Volvox.  Next  the  posterior  cells  of  the  blastaea  were 
thought  to  become  adapted  or  specialized  for  digestive  functions,,  the  assumption 
being  that  separation  of  the  digestive  and  locomotor  functions  would  have  an 
adaptive  advantage.  If  one  side  of  the  sphere  is  pushed  inward  or  invaginated — 
as  can  be  done  with  a  deflated  basketball,  for  example — so  that  it  comes  in 
contact  with  the  other  side,  a  pouchlike,  two-layered,  radially  symmetrical  struc- 
ture is  formed  that  approaches  the  basic  structure  of  the  coelenterates.  It  also  has 
the  form  of  the  two-layered  or  diploblastic  gastrula  stage  of  the  metazoan  embryo 
— whence  the  name,  gastraea,  of  this  hypothetical  organism. 


EVOLUTION    IN   ANIMALS  •   131 

The  two-layered  coelenterate  ancestors  were  then  supposed  to  have 
given  rise  to  the  flatworms  by  becoming  bilaterally  symmetrical  and  developing 
a  third  germ  layer,  the  mesoderm,  between  the  outer  ectoderm  and  the  endo- 
derm.  The  small  ciliated  planula  larva  of  the  coelenterates  has  been  compared 
with  the  ancestral  type  that  gave  rise  to  the  bilaterally  symmetrical  flatworms 
presumably  like  the  very  simple  ciliated  free-living  flatworms  of  the  order  Acoela 
of  the  class  Turbellaria.  The  appeal  of  the  theory  lies  in  its  synthesis  of  a  great 
deal  of  information  drawn  from  the  embryology  and  morphology  of  existing 
forms.  In  fact,  it  might  be  said  that  it  is  almost  too  good  to  be  true.  For  example, 
the  origin  of  the  internal  digestive  layer,  or  endoderm,  in  lower  forms,  is  gen- 
erally not  by  invagination  but  rather  through  the  inward  migration  of  many 
cells  from  the  ectoderm,  and  the  planula  larva  and  acoeloid  flatworms  have  an 
internal  solid  mass  of  cells  rather  than  being  hollow.  Other  criticisms  have  also 
been  directed  at  the  theory  as  outlined  above,  but  it  seems  likely  that  it  will  re- 
main a  strong  contender  for  some  time  to  come. 

2.  Another  suggestion  is  that  the  coelenterates,  like  the  sponges,  are 
off  the  main  path  of  metazoan  evolution  and  arose  independently  of  the  rest  of 
the  Metazoa.  The  flatworms  then  would  become  ancestral  to  the  higher  Metazoa. 
However,  the  presence  of  a  gastrovascular  cavity  in  both  coelenterates  and  flat- 
worms  and  of  a  mesogloea  between  the  ectoderm  and  the  endoderm  of  the 
coelenterates  comparable  to  the  mesoderm  of  the  flatworms  suggests  a  relation- 
ship between  them.  Furthermore,  the  Ctenophora,  while  not  necessarily  in  a 
direct  line  of  relationship  between  the  two  groups,  appear  to  show  some  similar- 
ities to  both. 

3.  Still  another  hypothesis  is  that  the  coelenterates  have  evolved  from 
the  flatworms  rather  than  vice  versa  as  in  the  gastraea  theory.  In  this  case  multi- 
nuclear  ciliates  were  postulated  to  give  rise  to  the  Turbellaria  Acoela  by  the 
formation  of  cells  around  the  nuclei.  From  the  Acoela  were  descended  the 
higher  Turbellaria  from  which  the  higher  invertebrates  arose  and  from  which 
the  coelenterates  and  the  ctenophores  were  separately  and  independently  evolved. 
On  this  view  bilateral  symmetry  was  the  primitive  condition,  and  the  radial  sym- 
metry of  the  coelenterates  was  a  secondary  development  associated  with  their 
sessile  mode  of  life. 

4.  Quite  a  different  concept  is  that  the  Metazoa,  except  for  their  mode 
of  nutrition,  are  more  like  multicellular  plants  than  like  Protozoa  and  that  the 
earliest  organisms  were  multinuclear  and  photosynthetic  plants,  which  were 
ancestral  to  the  Metazoa  and,  independently,  to  the  flagellates  and  the  other 
Protozoa. 

Although  other  theories  or  other  versions  of  the  above  theories  have 
been  advanced,  these  give  some  idea  of  the  diversity  of  opinion  on  the  subject. 
The  concept  followed  in  the  phylogenetic  chart  in  Fig.   13-1   is  that  of  the 


132  •  THE    EVIDENCE    FOR   EVOLUTION 

planula-acoela  line  of  descent,  not  only  because  it  is  currently  perhaps  the  most 
highly  regarded  of  the  various  possibilities  but  also  because  it  is  less  of  a  strain 
on  the  imagination.  One  reason  is  that  the  transition  from  radial  to  bilateral 
symmetry  can  be  more  readily  visualized.  This  change  was  a  major  one,  leading 
to  the  evolution  of  the  higher  phyla,  all  of  which  are  bilateral.  As  noted  above, 
however,  since  it  cannot  even  be  stated  with  assurance  that  the  change  was  in 
this  direction,  further  emphasis  on  the  origin  of  bilaterality  seems  unwarranted. 
However  the  stage  of  the  primitive  acoeloid  flatworms  may  have  been  reached,  a 
stage  similar  to  this  seems  very  likely  to  have  been  ancestral  to  the  higher  bi- 
lateral groups.  Although  again  all  of  the  relationships  among  the  various  phyla 
cannot  be  discerned,  two  major  lines  of  descent  can  be  recognized:  one,  the 
Protostomia,  leading  to  the  Arthopoda  and  Mollusca;  the  other,  the  Deutero- 
stomia,  leading  to  the  Chordata.  The  distinction  between  the  Protostomia  and 
the  Deuterostomia  is  based  on  their  mode  of  development.  In  the  Protostomia, 
the  mouth  forms  from  (or  in  the  region  of)  the  blastopore  whereas  in  the 
Deuterostomia  the  anus  forms  from  (or  in  the  region  of)  the  blastopore,  and 
the  mouth  is  formed  de  novo.  In  the  Protostomia,  furthermore,  embryonic  de- 
velopment typically  proceeds  by  spiral  cleavage  and  is  determinate;  that  is,  spe- 
cific cells  of  the  early  embryo  are  fated  to  give  rise  to  specific  parts  of  the  larva 
and  their  extirpation  results  in  a  deficient  larva.  The  trochophore  larva  character- 
istic of  this  group,  more  or  less  spherical  in  shape,  has  an  apical  tuft  of  cilia,  a 
ciliated  band  (the  prototroch)  at  the  equator,  and  a  complete  L-shaped  digestive 
tract. 

Nemertea 

The  flatworms  were  mentioned  earlier  as  lacking  a  coelom  or  body 
cavity,  and  one  other  phylum,  the  Nemertea  (also  known  as  Nemertinea  and 
Rhynchocoela)  or  ribbon  worms,  is  also  acoelomate.  They  resemble  the  flatworms 
in  several  respects,  having,  for  example,  a  ciliated  ectoderm  and  flame  bulbs  for 
excretion.  They  differ,  however,  in  having  a  complete  digestive  tract  with  mouth 
and  anus,  an  eversible  proboscis  not  connected  with  the  alimentary  canal,  and  a 
simple  blood  vascular  system,  differences  so  fundamental  that  assignment  to  a 
separate  phylum  seems  necessary. 

Acanthocephala 

A  fairly  large  number  of  groups  have  a  body  cavity  known  as  a  pseudo- 
coel,  since  it  lacks  the  mesodermal  lining  characteristic  of  the  coelom.  The  spiny- 
headed  worms  or  Acanthocephala  are  parasitic  as  larvae  in  various  arthropods 
and  as  adults  in  the  intestine  of  vertebrates.  Though  having  a  pseudocoel  and 
circular  as  well  as  longitudinal  muscles,  they  entirely  lack  a  digestive  tract,  the 


EVOLUTION    IN    ANIMALS  •   133 

retractable  proboscis  serving  as  an  organ  of  attachment  and  the  food  being 
directly  absorbed  from  the  host's  intestine.  The  excretory  organs  appear  to  be 
nephridia  with  modified  flame  bulbs,  and  in  some,  a  type  of  superficial  seg- 
mentation appears.  Although  these  traits  in  general  resemble  those  of  the  other 
pseudocoelomates  such  as  the  nematodes,  the  embryology  tends  to  resemble  that 
of  the  flatworms.  Therefore,  the  Acanthocephala,  even  though  a  small  group, 
have  generally  been  accorded  the  status  of  a  separate  phylum. 

The  next  six  groups  of  pseudocoelomate  animals  to  be  considered  show 
many  similarities  and  therefore  have  sometimes  been  placed  in  one  phylum,  the 
Aschelminthes.  These  groups,  which  here  are  treated  as  separate  phyla,  are  the 
Nematomorpha  (Gordiacea)  or  horsehair  worms,  the  Priapulida,  the  Kinor- 
hyncha  (Echinodera),  the  Nematoda  (Nemathelminthes)  or  roundworms,  the 
Gastrotricha,  and  the  Rotifera.  These  more  or  less  wormlike  animals  all  have  a 
complete  digestive  tract  with  a  posterior  anus. 

Nematoda 

Of  these  six  groups,  the  nematodes  include  by  far  the  largest  number  of 
species,  for  there  are  literally  thousands  of  free-living  and  parasitic  forms,  some 
of  an  extremely  unusual  nature;  one  species,  for  example,  has  been  found  only 
in  the  poison  gland  of  the  rattlesnake.  A  roundworm  is  a  rather  simply  con- 
structed animal.  In  addition  to  the  traits  noted  above,  the  body  is  covered  by  a  tough 
cuticle,  and  the  body  wall  has  only  a  single  layer  of  longitudinal  muscle  cells. 
There  are  no  respiratory  or  circulatory  organs,  and  the  excretory  system,  when 
present,  is  a  simple  canal  system  unlike  that  of  any  other  phylum.  The  nervous 
system  consists  of  a  circumenteric  ring  around  the  pharynx  and  a  simple  system 
of  associated  ganglia  and  nerves. 

Nematomorpha,  Kinorhyncha,  and  Priapulida 

The  Nematomorpha  are  much  like  the  nematodes  except  that  no  excre- 
tory system  is  present,  the  alimentary  canal  is  always  more  or  less  degenerate, 
and  there  is  just  a  single  ventral  nerve  cord.  The  long,  thin  adults,  thought  to 
resemble  "horsehair,"  are  free-living,  but  the  larvae  are  insect  parasites.  Another 
small  group,  the  Kinorhyncha  (Echinodera),  are  superficially  segmented  into 
13  or  14  rings  and  have  a  retractable  spiny  anterior  end.  There  are  two  excretory 
tubes  or  protonephridia  each  with  a  single  flame  bulb.  The  Priapulida,  with  only 
three  known  species,  are  also  superficially  segmented,  but  have  circular  as  well  as 
longitudinal  muscles.  The  spiny  retractile  anterior  end  calls  to  mind  the  kinor- 
hynchs,  as  does  the  type  of  nervous  system.  The  soft  posterior  processes  with 
gill-like  outgrowths  seem  to  be  unique.  The  excretory  system  consists  of  proto- 
nephridia and  solenocytes  (similar  to  flame  bulbs  except  that  they  have  a  single 


134  •  THE    EVIDENCE    FOR   EVOLUTION 

flagellum  rather  than  a  tuft  of  cilia).  Although  the  priapulids  have  been  grouped 
with  the  sipunculid  and  the  echiurid  worms  either  in  a  separate  phylum  Gephy- 
rea  or  else  as  a  class  of  annelids,  this  seems  clearly  in  error,  for  their  greatest 
affinities  are  with  the  kinorhynchs  and  nematodes,  and  they  also  show  certain 
traits  similar  to  those  of  the  rotifers  and  gastrotrichs. 

Gastrotricha  and  Rotifera 

Typical  gastrotrichs  are  minute  spiny  animals  that  glide  about  by  means 
of  ventral  cilia.  Each  lobe  of  the  forked  posterior  end  has  an  adhesive  gland  for 
temporary  attachment.  The  excretory  system  consists  of  paired  protonephridia 
each  with  a  single  flame  bulb.  The  rotifers  have  a  similar  excretory  system,  an 
anterior  retractile  ciliated  disc  or  corona,  and  a  posterior  forked  "foot"  with 
adhesive  glands.  The  internal  jaws  in  the  pharynx  are  unique  and  quite  distinc- 
tive. The  rotifers  are  generally  the  smallest  of  all  of  the  Metazoa. 

The  gastrotrichs  are  probably  closest  phylogenetically  to  the  nematodes, 
but  they  also  have  several  features  in  common  with  the  rotifers,  such  as  external 
cilia,  the  forked  foot,  and  the  excretory  system.  The  rotifers,  because  of  their 
resemblance  to  the  trochophore  larva  characteristic  of  the  annelid-mollusk  line 
of  descent,  are  thought  to  be  in  some  way  related  to  the  common  ancestor  of 
these  phyla.  However,  the  rotifers  also  resemble  the  free-living  flatworms,  per- 
haps more  than  they  do  any  other  group,  as  well  as  showing  affinities  with  the 
gastrotrichs  and  nematodes.  Hence  they  should  probably  be  regarded  as  a  group 
relating  the  turbellarian  flatworms  to  the  aschelminths. 

Entoprocta  and  Ectoprocta 

The  final  pseudocoelomate  phylum,  the  Entoprocta,  was  formerly 
placed  with  the  Ectoprocta  as  a  class  in  the  phylum  Bryozoa  (or  Polyzoa),  but 
the  resemblance  is  superficial.  The  entoprocts  have  a  pseudocoelom,  a  U-shaped 
digestive  tract  with  both  mouth  and  anus  opening  within  the  circle  of  tentacles, 
and  they  have  protonephridia  with  flame  bulbs  for  excretion.  The  ectoprocts,  a 
much  larger  group,  have  a  true  coelom  lined  with  mesoderm,  an  anus  that  opens 
outside  the  lophophore  bearing  ciliated  tentacles,  and  no  excretory  organs.  The 
similarities  lie  primarily  in  the  crown  of  tentacles  and  the  sessile  mode  of  life, 
which  is  usually  in  colonies.  However,  since  the  tentacular  crown  of  the  ento- 
procts is  not  comparable  or  homologous  to  the  lophophore  of  the  ectoprocts,  it 
is  clear  that  the  two  groups  should  be  separated.  The  group  nearest  the  ento- 
procts would  seem  to  be  the  rotifers.  Despite  the  many  well-defined  differences 
between  adult  entoprocts  and  ectoprocts,  both  types  develop  from  a  type  of  larva 
known  as  the  trochophore,  although  the  entoproct  larva  departs  in  some  respects 
from  the  typical  trochophore  larva. 


EVOLUTION    IN   ANIMALS  '135 

Among  the  animals  with  a  pseudocoel,  then,  are  six  groups  quite  clearly 
similar  and  two  phyla,  the  Acanthocephala  and  the  Entoprocta,  rather  different 
from  the  others.  Here,  too,  although  fundamental  morphological  similarities  exist 
that  clearly  seem  to  indicate  relationship,  the  exact  phylogenetic  sequence  is  ob- 
scured in  the  mists  of  the  past  and  may  never  be  known  with  certainty. 

Brachiopoda  and  Phoronida 

In  addition  to  the  Ectoprocta,  two  other  coelomate  phyla,  the  Brach- 
iopoda and  the  Phoronida,  also  have  a  lophophore,  and  these  three  phyla,  though 
quite  different  in  some  respects,  nevertheless  appear  to  be  related.  They  are 
similar  also  in  having  a  trochophore-like  larva  but  differ  in  that  both  phoronids 
and  brachiopods  have  a  simple  circulatory  system  and  an  excretory  system  with 
nephridia,  both  of  which  are  lacking  in  the  ectoprocts.  The  nephridial  system  of 
coelomate  invertebrates  is  typically  of  the  metanephridial  type,  in  which  the 
nephridial  tubules  begin  as  coelomic  openings,  draining  wastes  from  the  body 
cavity. 

Very  few  species  of  phoronids  are  known.  Sedentary,  wormlike  animals, 
they  are  all  marine,  living  in  a  self-secreted  tube  from  which  the  lophophore  is 
extended  to  feed.  The  brachiopods  or  lamp-shells  have  a  superficial  resemblance 
to  the  bivalve  mollusks  such  as  the  oyster,  but  the  two  halves  of  the  shell  are 
dorsal  and  ventral  rather  than  right  and  left  halves  as  in  the  bivalves.  An  unusual 
feature  of  brachiopod  development  is  the  formation  of  the  mesoderm  by  entero- 
coely  (out-pocketing  from  the  gut) ,  a  mode  of  mesoderm  formation  more  char- 
acteristic of  the  Deuterostomia  and  therefore  suggesting  affinities  with  the  echino- 
derms  and  chordates.  The  brachiopods  have  a  long,  extensive  fossil  record,  and 
the  living  species  represent  only  a  small  remnant  of  the  species  and  genera  of 
the  past.  One  living  genus,  Lingula,  has  persisted  virtually  unchanged  from  the 
Ordovician  period  of  the  Paleozoic,  some  400,000,000  years  ago,  and  is  therefore 
probably  the  oldest  living  genus. 

Mollusca 

The  Mollusca  are  the  second  largest  group  of  invertebrates,  having  five 
classes,  quite  diverse  in  appearance  but  with  an  underlying  fundamental  similar- 
ity. The  body  consists  of  a  head  (absent  in  bivalves  and  tooth  shells),  a  ventral 
muscular  foot,  and  a  dorsal  visceral  mass  covered  by  a  mantle,  which  usually 
secretes  a  calcareous  shell  on  its  upper  surface.  The  five  classes  are  as  follows : 

1.  Amphineura — chitons 

2.  Gastropoda — snails,  slugs,  limpets,  whelks,  abalone,  periwinkle,  conches,  etc. 

3.  Scaphopoda — tooth  shells 

4.  Pelecypoda — bivalves  such  as  clams,  oysters,  scallops,  and  mussels 

5.  Cephalopoda — nautili,  squids,  and  octopi 


136  •  THE    EVIDENCE    FOR   EVOLUTION 

The  radula,  a  rasping  organ  in  the  mouth  of  most  mollusks,  is  unique  to  the 
group,  and  here,  for  the  first  time,  we  encounter  respiratory  organs  either  in  the 
form  of  gills  (ctenidia)  or  lungs.  Though  mollusks  are  coelomate,  the  coelom  is 
reduced  to  the  cavities  of  the  gonads,  the  pericardium,  and  the  nephridia.  Both 
circulatory  and  excretory  systems  are  well  developed.  The  nervous  system  varies 
widely  from  the  simple  system  of  ganglia  in  bivalves  like  the  clam  to  the  com- 
plex centralized  system  with  a  "brain"  and  camera-type  eyes  of  cephalopods  such 
as  the  squid. 

The  mollusks  were  a  large,  well-defined  group  with  all  of  the  living 
classes  already  represented  at  the  beginning  of  the  Paleozoic.  The  trochophore 
larva  typical  of  many  mollusks  clearly  indicates  their  relationship  to  the  line  of 
descent  that  also  led  to  the  annelids  and  arthropods,  although  the  separation 
must  have  occurred  long  ago.  Most  of  the  Mollusca  show  little  or  no  evidence 
of  segmentation,  and  the  group  is  usually  referred  to  as  unsegmented.  However, 
the  recent  discovery  of  a  living  mollusk,  Neopilina  galatheae,  in  the  depths  off 
the  west  coast  of  Mexico  has  raised  serious  questions  as  to  whether  the  ancestral 
mollusks  were  segmented  (see  Fig.  13-2).  Neopilina  belongs  to  the  Amphineura, 
generally  presumed  to  be  closest  to  the  ancestral  mollusks  because  of  their  rela- 
tively simple  bilateral  structure  as  compared  with  the  other  classes  of  mollusks. 
Neopilina  has  five  pairs  of  small  gills,  and  each  gill  is  associated  with  a 
nephridium;  there  are,  furthermore,  five  pairs  of  dorso-ventral  muscles  associated 
with  the  foot.  Clearly,  this  arrangement  represents  well-defined  segmentation, 
and  the  possibility  must  now  be  admitted  that  ancestral  mollusks  were  seg- 
mented, the  modern  forms  representing  a  secondary  loss  of  the  segmented  con- 
dition. If  such  is  the  case,  then  the  mollusks  may  be  closer  to  the  annelids  than 
had  been  previously  suspected. 

Annelida 

The  members  of  the  phylum  Annelida,  to  which  belong  the  earthworms, 
polychaete  marine  worms,  and  leeches,  are  usually  conspicuously  segmented  both 
externally  and  internally,  with  the  body  composed  of  many  essentially  similar 
segments  or  somites.  This  segmentation  can  be  observed  not  only  in  the  append- 
ages and  muscles,  but  in  the  serial  repetition  of  the  parts  of  the  nervous,  excre- 
tory, circulatory,  and  reproductive  systems.  Each  somite  also  typically  bears  small 
rodlike  appendages  or  setae.  The  circulatory  system  consists  of  a  closed  system  of 
vessels  with  a  circulating  fluid  containing  a  respiratory  pigment.  The  larva,  when 
present,  is  a  trochophore,  and  the  early  development  of  annelids  and  mollusks  is 
quite  similar. 

Since  segmentation  is  present  in  the  two  dominant  phyla  of  animals  of 
the  present  time,  the  Arthropoda  and  the  Chordata,  it  must  represent  a  major 
evolutionary  advance.  However,  although  various  theories  of  the  origin  of  seg- 


EVOLUTION    IN    ANIMALS  •   137 


Fig.  13-2.  Neopilina  galatheae,  a 
recently  discovered  living  mollusk 
of  the  class  Amphineura,  with 
definite  signs  of  segmentation, 
suggesting  a  closer  relationship 
between  the  mollusks  and  the  seg- 
mented annelids  than  had  previ- 
ously been  suspected.  (With  per- 
mission of  Lemche.) 


mentation  have  been  advanced,  there  is  little  evidence  to  favor  any  one  theory 
over  the  rest  Furthermore,  segmentation  in  the  annelid-arthropod  line  appears  to 
have  arisen  independently  of  segmentation  in  the  chordates. 

Sipunculida  and  Echiurida 

The  sipunculid  and  echiurid  marine  worms  are  undoubtedly  related  to 
the  annelids  and,  perhaps  because  they  are  rather  small  groups,  have  sometimes 
been  classified  as  annelids.  Since  they  are  quite  different  from  the  earthworm  and 
other  annelids,  however,  more  recently  they  have  been  placed  in  separate  phyla. 
Both  Sipunculida  and  Echiurida  have  trochophore  larvae,  large  coeloms,  and 
somewhat  similar  circulatory  and  nephridial  systems.  The  sipunculid  or  "peanut" 
worms  are  gourd-shaped  with  a  narrow  retractile  anterior  end  crowned  with  a 
circle  of  ciliated  tentacles.  The  anus  is  anterior  and  dorsal.  The  echiurids  have  a 
troughlike  proboscis,  which  cannot  be  withdrawn  into  the  anterior  end  of  the 
body  like  that  of  the  sipunculids,  and  the  anus  is  posterior.  Bristlelike  setae  are 
present,  and  the  larvae  show  definite  signs  of  segmentation.  Thus  the  echiurids 
quite  definitely  belong  close  to  the  annelids. 


Onycophora,  a  Living  Link 

Whereas  the  evidence  for  the  relationships  among  the  various  groups 
presented  thus  far  has  been  rather  tenuous  in  most  cases,  the  evidence  for  the 
relationship  between  annelids  and  arthropods  is  much  more  clearcut.  These 
phyla  show  many  similarities  both  in  mode  of  development  (although  a  tro- 
chophore larva  is  absent  in  arthropods)  and  in  adult  structure.  The  arthropods 


138  •  THE    EVIDENCE    FOR   EVOLUTION 

differ  from  annelids  in  having  a  thick  chitinous  exoskeleton,  jointed  appendages, 
and  muscles  in  functional  groups  rather  than  simple  continuous  sheets.  The 
coelom  of  the  arthropods  is  much  reduced  and  is  more  or  less  replaced  by  the 
haemocoele  of  the  circulatory  system,  and  the  excretory  and  reproductive  systems 
are  concentrated  rather  than  segmental. 


Fig.  13-3.     Peripatus     (Macroperipatus    geayi)     of    the    phylum    Onycophora,    a 
connecting  link  between  the  annelids  and  the  arthropods.    (Photo  by  Ralph  Buchs- 

baum.) 


These  two  phyla  are  the  only  major  invertebrate  groups  with  con- 
spicuous true  segmentation.  One  other  small  phylum,  the  Onycophora,  is  also 
segmented,  and  has  a  unique  mixture  of  annelid  and  arthropod  traits.  They  are 
like  annelids  in  having  segmental  nephridia,  simple  eyes  but  no  well-defined 
head,  a  soft  cuticle,  short  un jointed  appendages,  and  muscles  in  continuous 
sheets.  Arthropod  traits  include  the  reduced  coelom  with  the  haemocoele  as  the 
adult  body  cavity,  the  tracheal  respiratory  system,  and  the  circulatory  system  with 
a  dorsal  "heart."  The  Onycophora,  represented  by  Peripatus  (Fig.  13-3),  have 
been  classed  with  the  arthropods  and  also  as  annelids,  but  it  seems  best  to  place 
them  for  the  present  in   a  separate  phylum,   for  their  features,   although  re- 


EVOLUTION    IN    ANIMALS  •   139 

sembling  those  in  both  groups,  are  different  enough  to  suggest  that  the  Onyco- 
phora  are  a  very  old  group.  Rather  than  being  a  missing  link  between  Annelida 
and  Arthropoda,  they  perhaps  represent  a  third  independent  line  of  descent  from 
the  ancestral  stock  that  gave  rise  to  modern  annelids  and  arthropods.  In  any 
event  their  very  existence  tends  to  reinforce  the  postulated  relationship  between 
those  two  phyla. 

Arthropoda 

The  Arthropoda  have  by  far  the  greatest  number  of  species  of  any 
phylum.  The  following  classes  have  been  recognized,  most  of  them  including 
very  familiar  forms. 

1.  Trilobita — extinct  aquatic  forms 

2.  Crustacea — shrimps,  copepods,  crabs,  lobsters,  etc. 

3.  Arachnida — spiders,  ticks,  mites,  scorpions,  horseshoe  crabs,  eurypterids  (ex- 

tinct), etc. 

4.  Myriapoda — centipedes,  millipedes 

5.  Insecta — butterflies,  beetles,  bees,  dragonflies,  etc. 

The  Arthropoda  may  be  described  as  segmented  animals  with  jointed  append- 
ages, a  haemocoele,  and  a  thick  chitinous  exoskeleton.  This  body  plan  has  been 
enormously  successful  in  all  sorts  of  habitats.  Different  species  have  adapted  to 
life  in  the  depths  of  the  sea,  on  land,  and  in  the  air.  The  exoskeleton  undoubt- 
edly made  possible  the  invasion  of  the  land  by  protecting  the  animals  against 
dessication,  and,  by  providing  rigid  points  of  attachment  for  the  muscles,  it  also 
is  related  to  their  speed  of  movement.  Furthermore,  the  great  morphological 
specialization  and  diversification  of  the  exoskeleton  into  various  types  of  legs, 
wings,  and  mouth  parts  has  made  possible  adaptation  to  a  great  variety  of  eco- 
logical niches. 

Chaetognatha  and  Pogonophora 

The  phyla  remaining  to  be  considered,  Chaetognatha,  Echinodermata, 
Pogonophora,  Hemichordata,  and  Chordata,  all  belong  to  the  Deuterostomia. 
The  arrow  worms  or  Chaetognatha  resemble  in  the  simplicity  of  their  structure 
(no  excretory,  respiratory  or  circulatory  systems)  some  of  the  pseudocoelomate 
groups.  However,  they  have  a  large  true  coelom  and  their  early  embryology  re- 
sembles that  of  the  echinoderms  and  chordates.  A  post-anal  tail  is  found  only  in 
this  group  and  among  the  chordates.  The  bristles  about  the  mouth,  from  which 
the  phylum  gets  its  name,  aid  in  the  capture  of  food.  Although  the  arrow  worms 
appear  to  belong  among  the  Deuterostomia,  they  show  no  obvious  relation  to 
any  other  members  of  this  group.  The  Pogonophora,  sedentary  worms  living  in 


140  •  THE    EVIDENCE    FOR   EVOLUTION 

long  tubes  in  the  depths  of  the  Pacific,  were  originally  thought  to  be  polychaete 
annelids,  but  more  recently  they  have  been  placed  in  a  separate  phylum  with 
their  closest  affinities  to  the  Hemichordata.  Because  of  the  complex  tentacles  at 
the  anterior  end,  somewhat  like  the  lophophore  of  the  phoronids,  ectoprocts, 
and  brachiopods,  they  have  been  placed  between  the  hemichordates  and  the 
lophophorates.  However,  the  exact  status  of  this  group  will  not  be  well  estab- 
lished until  it  has  been  more  extensively  studied. 

Echinodermata 

The  Echinodermata,  which  include  such  species  as  starfish,  crinoids, 
brittle  stars,  sea  urchins,  and  sea  cucumbers,  have  ciliated,  free-swimming,  bi- 
laterally symmetrical  larvae  and  radially  symmetrical  adults,  presumably  a  sec- 
ondary development  related  to  the  adults'  sessile  mode  of  life.  Although  a  star- 
fish is  a  far  cry  from  a  vertebrate,  nevertheless  the  echinoderms,  hemichordates, 
and  chordates  clearly  form  a  related  group.  The  relationship  is  based  primarily 
on  the  similarities  in  their  embryological  development.  In  the  Deuterostomia  not 
only  is  the  mouth  newly  formed,  the  blastopore  becoming  the  anus,  but  cleavage 
is  indeterminate,  and  the  mesoderm  and  the  coelom  originate  from  pouches 
formed  from  the  wall  of  the  primitive  gut  (enterocoely) .  Furthermore,  the 
echinoderm  skeleton  is  derived  from  the  mesoderm  as  it  is  in  the  chordates,  un- 
like its  mode  of  origin  in  any  other  invertebrate  group.  The  different  groups  of 
echinoderms  have  several  distinctive  types  of  larvae,  but  in  the  early  stages  of 
development  all  echinoderm  larvae  pass  through  a  dipleurula  stage  during  which 
they  show  several  traits  in  common.  The  dipleurula  larvae  are  bilaterally  sym- 
metrical, swim  by  means  of  longitudinal  looped  ciliated  bands,  and  have  an 
anterior  coelom  that  opens  to  the  dorsal  surface  through  a  pore.  There  is  an 
anterior  tuft  of  sensory  cilia,  a  ventral  mouth,  and  a  posterior  anus.  The  develop- 
ing Hemichordata  pass  through  stages  very  similar  to  the  dipleurula  larva,  and 
the  tornaria  larvae  of  the  hemichordate  tongue  worms  are  so  similar  to  the 
bipinnaria  larvae  of  the  starfishes  that  they  were  originally  described  as  starfish 
larvae  (Fig.  13-4).  These  larvae  and  their  mode  of  development  are  so  different 
from  the  trochophore  larva  characteristic  of  the  mollusk-annelid  line  that  the 
larval  traits  have  served  as  the  basis  for  the  diphyletic  system  of  evolution  de- 
scribed here.  Although  larval  resemblances  and  differences  may  be  misleading 
because  the  larvae  themselves  may  evolve  in  adapting  to  their  environments,  the 
differences  between  dipleurula  and  trochophore  larvae  appear  to  be  more  funda- 
mental than  can  be  accounted  for  by  differing  adaptive  responses.  Finally,  it 
should  be  noted  that  the  larvae  of  these  and  other  forms  are  best  interpreted  as 
recapitulating  the  larvae  of  the  ancestral  forms  rather  than  as  being  representa- 
tive of  the  adult  ancestor. 

Adult  echinoderms  have  unsegmented  bodies  usually  with  five  arms  (or 
multiples  of  five)  bearing  tube  feet.  The  water  vascular  system,  of  which  the 


EVOLUTION    IN   ANIMALS  •   141 


Mouth 


Adult  ACORN  WORM 
Fig.  13-4.     Larval  homology  in  the  echinoderms  and  the  hemichordates. 

tube  feet  form  a  part,  is  a  unique  system  for  locomotion,  respiration,  and  food 
handling.  The  digestive  system  is  complete  though  the  anus  is  small  (in  some  it 
is  lacking),  and  the  coelom  is  well  developed.  Nervous  and  circulatory  systems, 
though  present,  are  reduced.  Among  all  the  invertebrates,  the  starfish  and  its  kin 
seem  very  unlikely  candidates  as  relatives  to  the  phylum  that  we,  at  least,  tend 
to  regard  so  highly,  the  chordates. 

Hemichordata 

The  hemichordates  have  sometimes  been  classified  as  a  subphylum  of 
the  Chordata,  but  more  recently  the  trend  has  been  to  call  them  a  separate 
phylum.  Small  wormlike  animals,  they  have  indications  of  the  three  chordate 


142  •  THE    EVIDENCE    FOR   EVOLUTION 

traits — notochord,  pharyngeal  gill  slits,  and  dorsal  nerve  cord — but  in  each  case 
some  doubt  exists  as  to  their  homology.  The  body  is  composed  of  a  proboscis,  a 
collar,  and  a  trunk,  each  region  having  separate  coelomic  cavities.  The  mouth 
opens  at  the  anterior  margin  of  the  collar  into  the  digestive  tract,  and  just  back 
of  the  collar  numerous  gill  slits  permit  excess  water  to  pass  out  of  the  tract. 
There  is  some  question  as  to  whether  the  gill  slits  have  a  respiratory  function.  The 
"notochord"  or  stomocord  projects  forward  into  the  proboscis  as  an  anterior  out- 
pocketing  of  the  digestive  tract  and  serves  as  a  supporting  structure,  but  whether 
it  is  truly  homologous  to  the  notochord  is  doubtful.  Since  the  ventral  nerve  cord 
is  more  extensive  than  the  dorsal  one  that  is  limited  to  the  collar,  again  the 
homologies  are  not  clear.  Thus,  although  the  acorn  worms  are  clearly  more  like 
the  chordates  than  like  any  other  group,  they  are  still  sufficiently  different  to  be 
considered  as  a  separate  phylum. 

An  extinct  group  known  as  the  graptolites  has  recently  been  included 
among  the  Hemichordata,  but  the  evidence  for  this  relationship  is  rather  tenu- 
ous, and  further  information  seems  necessary  before  any  well-founded  conclu- 
sions can  be  drawn. 

Chordata 

The  phylum  Chordata  has  three  subphyla: 

1.  Urochordata  or  Tunicata — the  tunicates  or  sea  squirts  or  ascidians 

2.  Cephalochordata — amphioxus  or  the  lancelets 

3.  Vertebrata — the  back-boned  animals  or  vertebrates 

The  sessile  adult  tunicate  shows  little  to  suggest  its  affinity  to  the  other  chordates, 
but  the  free-living  larvae  clearly  show  chordate  characteristics.  The  notochord  of 
the  larva  is  confined  to  the  tail  (whence  the  name  Urochordata).  The  dorsal 
hollow  nerve  cord  terminates  anteriorly  in  a  "brain"  and  a  median  eye.  Gill 
slits  are  found  in  a  region  comparable  to  the  pharynx  in  the  higher  chordates, 
and  thus  all  three  traits  are  clearly  present.  Upon  settling  down,  the  larva  has  its 
tail  reabsorbed,  the  notochord  disappears,  and  the  nervous  system  is  reduced  to 
a  ganglion.  The  gill  slits  are  incorporated  into  a  large  branchial  sac,  and  a  test 
or  tunic  is  secreted  over  the  outer  surface. 

In  the  cephalochordates  the  three  distinctive  chordate  traits  are  seen  in 
simple  form  in  the  adults.  The  notochord  and  dorsal  nerve  cord  extend  the 
length  of  the  body  up  into  the  anterior  tip  (hence  the  name  Cephalochordata, 
even  though  they  have  no  distinct  head).  Numerous  gill  arches  associated  with 
the  circulatory  system  are  found  in  the  pharyngeal  region.  Amphioxus  is  a  fre- 
quent subject  of  study  in  zoology,  for  the  circulatory,  muscular,  nervous,  and 
other  systems  are  thought  to  be  representative  of  the  ancestral  chordate  condi- 
tion. The  presence  of  nephridia  that  appear  to  resemble  those  of  certain  poly- 
chaete  annelid  worms  constitutes  something  of  a  phylogenetic  puzzle. 


EVOLUTION    IN    ANIMALS  •   143 

The  vertebrates,  whose  evolution  has  already  been  discussed,  are  the 
dominant  animals  on  the  earth  at  present,  for  to  this  group  belong  the  fishes  of 
the  sea,  the  mammals  on  the  land,  and  the  birds  of  the  air.  Although  many 
species  as  adults  lack  gills  and  a  notochord,  nevertheless  at  some  stage  in  the 
life  cycle  the  basic  chordate  traits  appear  and  the  relationship  of  all  of  these 
groups  to  one  another  is  clearly  evident. 

Confronted  by  the  great  diversity  of  species,  one  well  can  wonder 
whether  it  is  possible  to  decipher  any  sort  of  orderly  relationship  among  so  many 
thousands  of  kinds  of  animals.  The  surprising  thing  perhaps  is  not  that  so  few 
well-defined  relationships  have  been  pinned  down,  but  rather  that  the  phylogeny 
of  the  animal  kingdom  is  as  well  known  as  it  is.  When  the  great  gaps  in  our 
knowledge  of  the  past  are  realized,  it  is  easier  to  appreciate  the  problems  in- 
volved. One  other  factor  that  is  almost  impossible  for  the  human  mind  to  en- 
compass is  the  vast  stretch  of  time  available  in  the  past  during  which  some  of  the 
otherwise  almost  unbelievable  evolutionary  changes  took  place.  If  the  magnitude 
of  the  evolutionary  changes  of  just  the  past  ten  million  years  can  be  appreciated, 
it  becomes  perhaps  somewhat  easier  to  comprehend  the  magnitude  of  changes 
possible  during  periods  ranging  up  to  hundreds  of  millions  of  years. 

— ►  SUMMARY 

Any  survey  of  the  animal  kingdom  tends  to  stress  the 
means  of  distinguishing  the  different  kinds  of  animals  from  one 
another,  but  it  must  be  remembered  that  all  animals  share  many 
traits  in  common.  Furthermore,  despite  many  questionable  or 
dubious  points,  it  is  possible  to  work  out  a  phylogeny  of  the 
animal  kingdom  based  on  the  similarities  among  the  different 
groups.  Although  such  a  phylogeny  is  based  on  the  assumption  of 
evolution,  the  very  fact  that  the  phylogeny,  when  constructed, 
forms  a  branching  system  is  in  itself  an  argument  favoring 
evolution. 

SUGGESTED  READING 

Berrill,  N.  J.,  1955.  The  origin  of  vertebrates.  Oxford:  Clarendon  Press. 

Borradaile,  L.  A.,  and  F.  A.  Potts,  1958.  The  Invertebrata,  3d  ed.  New  York: 
Macmillan. 

Buchsbaum,  R.,  1948.  Animals  without  backbones,  2d  ed.  Chicago:  University  of 
Chicago  Press. 

de  Beer,  G.  R.,  1954.  The  evolution  of  metazoa.  Evolution  as  a  process.  J.  Huxley, 
A.  C.  Hardy,  and  E.  B.  Ford,  eds.  London:  Allen  and  Unwin. 

Hyman,  L.  H.,  1940-1959.  The  invertebrates,  Vols.  1-5.  New  York:  McGraw-Hill. 

Marcus,  E.,  1958.  "On  the  evolution  of  animal  phyla,"  Quart.  Rev.  Biol.,  33/24-58. 

Storer,  T.  I.,  and  R.  L.  Usinger,  1957.  General  zoology,  3d  ed.  New  York:  McGraw- 
Hill. 

Young,  J.  Z.,  1950.  The  life  of  vertebrates.  Oxford:  Clarendon  Press. 


CHAPTER 


U 

Evolution  in  Plants 


In  the  plant  kingdom  as  in  the  animal  kingdom  classi- 
fication has  been  attempted  in  a  way  that  conforms  with  the 
postulated  phylogeny  of  the  various  groups.  This  effort  has  been 
only  partially  successful,  for  again  in  many  cases  the  relationships 
are  difficult  to  decipher  and  arbitrary  decisions  have  been  neces- 
sary. However,  because  additional  research  seemed  to  indicate  that 
the  existing  classification  did  not  accurately  reflect  the  relation- 
ships among  the  various  plants,  a  major  revision  in  classification 
of  the  plant  kingdom  was  recently  made.  The  classical  classifica- 
tion was  as  follows: 

Kingdom  Plantae 

Division  Thallophyta 

Subdivision  Algae — seaweeds,  kelps,  pond  scum,  etc. 
Subdivision  Fungi — molds,  yeasts,  bacteria,  mushrooms,  etc. 
Division  Bryophyta 

Class  Hepaticae — liverworts 
Class  Musci — mosses 
Division  Pteridophyta 

Class  Filicineae — ferns 
Class  Equisetineae — horsetails 
Class  Lycopodineae — club  mosses 
Division  Spermatophyta 

Subdivision  Gymnospermae — conifers 
Subdivision  Angiospermae — flowering  plants 
Class  Dicotyledoneae 
Class  Monocotyledoneae 


144 


EVOLUTION    IN    PLANTS  •   145 

The  more  modern  classification,  which  has  been  based  on  recent  mor- 
phological and  paleobotanical  work  and  is  believed  to  be  a  more  natural  system, 
is  as  follows: 


formerly 
Algae 


formerly 
Fungi 


formerly 
Pteridophyta 

formerly 
Pteridophyta 

formerly 
Spermatophyta 


Kingdom  Plantae 

Phylum*  Cyanophyta — blue-green  algae 

Phylum  Euglenophyta — euglenoids 

Phylum  Chlorophyta — green  algae 

Phylum  Chrysophyta — yellow-green  and  golden  brown 

algae  and  diatoms 
Phylum  Pyrrophyta — cryptomonads  and  dinoflagellates 
Phylum  Phaeophyta — brown  algae 
Phylum  Rhodophyta — red  algae 

Phylum  Schizomycophyta — bacteria 
Phylum  Myxomycophyta — slime  molds 
Phylum  Eumycophyta — true  fungi 

Phylum  Bryophyta — mosses,  liverworts,  and  hornworts 

Phylum  Tracheophyta — vascular  plants 

SUBPHYLUM   PSILOPSIDA 

Subphylum  Lycopsida — club  mosses 
Subphylum  Sphenopsida — horsetails 

Subphylum  Pteropsida 
Class  Filicineae — ferns 

Class  Gymnospermae — conifers 
Class  Angiospermae — flowering  plants 


The  major  changes  can  be  seen  to  be  an  upgrading  in  the  systematic 
rank  of  the  various  algae,  reflecting  the  belief  that  these  groups  are  not  at  all 
closely  related,  and  a  rearrangement  in  the  classification  of  the  different  groups 
of  higher  plants.  The  latter  change  seemed  necessary  because  recent  evidence  has 
tended  to  break  down  some  of  the  former  distinctions  between  the  pteridophytes 
and  the  spermatophytes. 

The  terms  thallophyte,  algae,  and  fungi  are,  however,  useful  ones  and 
undoubtedly  will  continue  to  be  used  even  though  it  is  recognized  that  they 
represent  artificial  groupings.  The  phyla  considered  as  thallophytes  are  plants 
that  lack  true  roots,  stems,  and  leaves  (or  to  be  more  specific,  the  vascular  tissues, 
xylem  and  phloem),  and  in  which  the  zygote  does  not  form  a  multicellular 


*  The  Botanical  Rules  of  Nomenclature  recognize  "Divisions"  rather  than  "Phyla,"  but 
the  latter  term  is  used  here  to  parallel  zoological  usage. 


146  •  THE    EVIDENCE    FOR   EVOLUTION 

embryo  while  still  in  the  female  sex  organs.  The  algae  are  thallophytes  possessing 
chlorophyll;  the  fungi  are  thallophytes  lacking  chlorophyll.  The  postulated  rela- 
tionships among  the  different  plant  phyla  are  shown  in  Fig.  14-1. 

Cyanophyta 

The  phylum  Cyanophyta  (or  blue-green  algae)  is  an  extremely  primi- 
tive group.  The  plant  is  a  single  cell,  occasionally  grouped  in  loose  aggregations. 
There  apparently  is  no  definite  nucleus,  for  the  chromatin  appears  scattered  in 
the  center  of  the  cell.  The  chlorophyll  is  diffused  rather  than  being  organized 
into  plastids.  The  blue  color  is  due  to  another  pigment,  phycocyanin,  and  a  red 
pigment  may  also  be  present.  The  only  known  method  of  reproduction  is  by 
asexual  fission,  and  none  of  the  cells  of  the  blue-greens  has  flagella.  The  Cyano- 
phyta have  been  described  from  Precambrian  rocks  estimated  to  be  a  billion  years 
old  and  are,  therefore,  among  the  oldest  known  fossil  plants. 

Rhodophyta 

The  phylum  Rhodophyta  (or  red  algae)  takes  its  name  from  the  red 
pigment  phycoerythrin  associated  in  the  plastids  with  chlorophyll  and  also  in 
some  species  with  phycocyanin.  The  thallus  is  ordinarily  multicellular,  composed 
of  nucleated  cells.  The  life  cycle  may  be  complex,  with  both  sexual  and  asexual 
reproduction,  but  an  unusual  feature  of  these  algae  is  the  absence  of  any  type  of 
flagellated  reproductive  cell.  The  red  algae  have  a  fossil  record  going  back  to  the 
Ordovician  and  show  little  resemblance  to  any  other  algal  group  except  the  blue- 
greens.  Both  groups  lack  flagellated  cells  and  have  in  common,  in  at  least  some 
species  of  both  groups,  the  red  and  blue  pigments,  phycoerythrin  and  phy- 
cocyanin. 

Pyrrophyta  and  Chrysophyta 

The  cryptomonads  and  dinoflagellates  have  been  placed  by  botanists  in 
the  phylum  Pyrrophyta.  Most  members  of  this  phylum  are  unicellular  with  two 
unlike  flagella,  yellow-green  to  golden-brown  plastids,  no  cell  walls,  and  reserve 
food  in  the  form  of  starches  or  oils. 

The  Chrysophyta  include  the  yellow-green  algae,  the  golden  brown 
algae,  and  the  diatoms.  The  name  chrysos,  "golden,"  stems  from  the  fact  that 
there  are  more  yellow  or  brown  carotenoid  pigments  than  there  is  chlorophyll, 
with  both  pigments  being  found  in  plastids.  The  food  reserves  are  oils  and 
leucosin,  an  insoluble  carbohydrate.  The  cell  walls  are  usually  formed  of  over- 
lapping halves,  frequently  silica  impregnated.  The  three  classes  of  this  phylum, 
in  some  ways  quite  different,  are  thought  to  be  related  because  of  the  similar 


METAZOA 

Other 

PROTOZOA 


SCHIZOMYCOPh 
(Bacteria) 


©4©  ® 


CYANOPHYTA 
(Blue-green  algae) 


F*#.  24-1.     The  phylogeny  of  the  plant  kingdom. 


148  •  THE    EVIDENCE    FOR   EVOLUTION 

types  of  reserve  food  and  the  silicified  bipartite  cell  walls.  The  Pyrrophyta  and 
Chrysophyta  show  some  affinities,  but  the  phylogenetic  relationships  of  these  two 
groups  are  still  far  from  clear. 

Phaeophyta 

The  brown  algae  or  Phaeophyta  have  their  photosynthetic  pigments 
masked  by  the  brown  pigment,  fucoxanthin.  The  plants  are  multicellular,  ranging 
in  size  from  a  few  cells  to  the  giant  kelps  over  100  feet  in  length,  and  are 
vegetatively  the  most  highly  specialized  group  among  all  of  the  algae.  Not  only 
may  the  plant  bodies  be  highly  differentiated,  but  a  variety  of  methods  of  repro- 
duction have  evolved,  and  there  is  commonly  an  alternation  of  generations. 
Although  the  brown  algae  have  become  the  most  advanced  in  structure  among 
the  algae,  resembling  in  some  respects  the  primitive  vascular  plants,  they  are  not 
thought  to  have  given  rise  to  any  higher  groups  of  plants  nor  are  they  considered 
to  be  very  closely  related  to  any  other  group  of  algae. 

Euglenophyta  and  Chlorophyta 

Almost  all  of  the  Euglenophyta  are  naked  unicellular  flagellates  with 
the  chlorophyll  not  associated  with  any  other  pigments  except  the  usual  caroti- 
noids  (carotene  and  xanthophyll)  found  in  the  green  algae  and  the  higher 
plants.  They  differ  from  the  blue-green  algae  in  having  the  reserve  food  in  the 
form  of  the  carbohydrate,  paramylum,  and  fats. 

The  green  algae  or  Chlorophyta  have  chlorophyll  and  the  associated 
carotenoids  in  the  same  proportions  as  the  higher  plants.  The  cells  have  definite 
nuclei  and  chloroplasts,  are  often  flagellated,  and  the  thallus  may  be  unicellular, 
multicellular,  or  colonial.  The  reserve  food  is  starch,  and  cellulose  cell  walls  are 
present;  in  these  respects  the  green  algae  differ  from  the  euglenoids.  However, 
the  green  algae  are  clearly  rather  similar  to  the  euglenophytes  and  are  thought 
to  have  been  derived  from  them.  Furthermore,  both  the  bryophytes  and  the  vas- 
cular plants  are  considered  to  have  evolved  from  filamentous  green  algae. 

Schizomycophyta 

Although  the  bacteria  (Schizomycophyta)  show  some  structural  and 
reproductive  similarities  to  the  blue-green  algae  and  to  some  of  the  true  fungi, 
their  exact  phylogenetic  position  is  unknown  and  will  probably  remain  a  matter 
of  speculation.  They  are  extremely  small  (up  to  5  microns)  and  structurally 
simple  unicellular  organisms.  Bacteria  are  generally  believed  to  have  been  among 
the  first  living  organisms  on  earth.  Most  bacteria  are  parasites  or  saprophytes 
(obtaining  food  from  nonliving  organic  matter),  and  are  called  heterotrophic. 
However,  some  bacteria,  such  as  iron  and  sulfur  bacteria,  are  autotrophic — that 


EVOLUTION   IN    PLANTS  •   149 

is,  capable  of  synthesizing  organic  compounds  from  simple  inorganic  substances. 
Some  of  the  rich  iron  ore  deposits  of  the  earth  are  extremely  old  and  are  thought 
to  have  been  formed  by  the  action  of  iron  bacteria,  which  obtain  the  necessary 
energy  for  organic  syntheses  from  the  oxidation  of  ferrous  compounds  in  iron- 
bearing  waters.  Thus,  these  autotrophic  chemosynthetic  bacteria  could  have  ex- 
isted even  before  the  photosynthetic  process  had  evolved.  Furthermore,  since 
evidence  is  accumulating  as  to  ways  in  which  organic  compounds  could  have 
been  synthesized  by  nonliving  systems  under  different  environmental  conditions 
in  the  distant  past,  it  is  conceivable  that  heterotrophic  bacteria  could  also  have 
preceded  photosynthetic  organisms.  Some  bacteria  are  photosynthetic,  and  the 
bacteria  have  been  suggested  as  possible  progenitors  for  both  the  algae  and  the 
fungi.  However,  this  hypothesis  is  by  no  means  well  established,  and  it  has  also 
been  suggested  that  the  three  groups  have  evolved  in  parallel  from  an  unknown 
common  ancestor  or  even  that  the  bacteria  are  a  degenerate  rather  than  a  primi- 
tive group.  The  latter  hypothesis  seems  to  have  less  evidence  in  its  favor,  and 
the  current  tendency  is  to  regard  the  bacteria  as  truly  primitive  plants,  but  their 
exact  relationships  to  other  microorganisms  and  plant  groups  are  likely  to  remain 
obscure. 

Myxomycophyta  and  Eumycophyta 

The  slime  molds  or  Myxomycophyta  are  typically  saprophytes  with  an 
unusual  life  cycle  that  includes  both  animal  and  plantlike  features.  The  organism 
consists  of  a  naked  multinucleate  protoplasmic  mass  or  plasmodium,  which 
creeps  slowly  about  in  an  amoeboid  fashion  and  is  capable  of  ingesting  solid 
food  particles.  Under  favorable  conditions,  the  plasmodium  ceases  to  move  and 
forms  spore-bearing  fruiting  bodies  or  sporangia,  characteristic  of  plants.  The 
affinities  of  the  slime  molds  are  uncertain,  for  they  appear  to  be  transitional 
forms  between  the  plant  and  animal  kingdoms.  In  some  respects  they  seem  more 
closely  related  to  certain  protozoa  than  to  any  other  groups,  yet  they  also  show 
similarities  to  the  more  primitive  true  fungi  or  Eumycophyta. 

The  true  fungi  are  quite  a  diverse  group.  Common  to  all  of  the  Eumy- 
cophyta is  their  heterotrophic  nutrition  and  their  ability  to  produce  spores,  and 
most  of  them  have  plant  bodies  consisting  of  masses  of  filaments  or  hyphae. 
Three  suggestions  have  been  made  as  to  the  origin  of  the  true  fungi.  They  show 
some  resemblance  to  the  Myxomycophyta,  to  certain  Protozoa,  and  also  to  some 
of  the  algae,  from  which  they  might  have  arisen  through  loss  of  chlorophyll. 
However,  again  the  exact  phylogeny  is  unknown. 

Overlapping  Systems  of  Classification 

At  this  point  it  may  be  well  to  stop  and  reassess  some  of  the  material 
just  covered,  for  there  is  a  fundamental  inconsistency  that  needs  to  be  brought 
out  in  further  detail.  The  systems  of  classification  for  the  plant  and  animal  king- 


150  •  THE    EVIDENCE    FOR   EVOLUTION 

doms  that  have  been  outlined  above  are  rather  generally  used  and  are  widely 
accepted  by  botanists  and  by  zoologists.  However,  in  some  respects,  these  group- 
ings into  plant  and  animal  phyla  are  deceptively  clear-cut.  For  example,  most 
zoologists  classify  the  groups  known  as  cryptomonads,  chrysomonads,  phyto- 
monads,  chloromonads,  euglenoids,  and  dinoflagellates  in  the  class  Flagellata  of 
the  phylum  Protozoa.  Most  botanists,  on  the  other  hand,  regard  cryptomonads 
and  dinoflagellates  as  members  of  the  algal  phylum  Pyrrophyta,  chrysomonads  as 
members  of  the  phylum  Chrysophyta,  euglenoids  as  Euglenophyta,  and  phyto- 
monads  (or  Volvocales)  and  chloromonads  as  Chlorophyta.  Furthermore,  some 
zoologists  consider  the  slime  molds,  which  botanists  classify  as  the  phylum 
Myxomycophyta,  to  be  an  order,  the  Mycetozoa,  of  the  class  Sarcodina  (or 
Rhizopoda)  of  the  phylum  Protozoa.  These  differences  are  not  altogether  the 
result  of  chauvinistic  tendencies  of  the  two  groups  of  scientists,  but  rather  reflect 
the  fact  that  it  is  virtually  impossible  to  draw  a  well-defined  line  between  ani- 
mals and  plants.  Clearly,  since  the  zone  of  overlap  is  so  broad,  all  living  things 
belong  to  one  great  interrelated  system,  and  the  separation  into  plant  and  animal 
kingdoms  must  be  regarded  as  a  convenient  but  artificial  device. 

Another  approach  to  this  problem  has  been  the  creation  of  a  third 
kingdom,  the  Protista,  in  addition  to  Animalia  and  Plantae.  Included  in  the 
Protista  are  such  groups  as  the  bacteria,  the  protozoa,  and  the  slime  molds. 
Although  this  system  has  some  merit,  in  that  some  of  the  duplication  can  be 
avoided,  it  has  the  drawback  that  two  artificial  lines  are  required  rather  than  one. 
However,  it  is  to  be  hoped  that  in  time  the  historical  barriers  between  botany  and 
zoology  will  gradually  erode,  and  a  generally  accepted  biological  system  of  classi- 
fication for  the  lower  organisms  will  emerge,  which  will  lack  some  of  the  diffi- 
culties of  the  system  now  in  use.  At  the  present  time  there  is  no  generally  ac- 
cepted system  of  classification  covering  all  living  things.  Although  this  discovery 
may  be  disconcerting  to  the  beginning  biology  student  who  likes  to  have  things 
neatly  packaged  with  no  loose  ends,  to  the  student  of  evolution  it  should  come 
as  no  surprise,  for  it  tends  to  confirm  the  validity  of  the  theory  of  evolution. 

No  mention  of  the  phylogenetic  position  of  the  viruses  has  been  made 
thus  far,  simply  because  there  is  virtually  nothing  to  say.  The  viruses  consist  of 
the  hereditary  material,  DNA  (deoxyribonucleic  acid,  or  in  some  cases  RNA, 
ribonucleic  acid)  covered  by  a  protein  sheath,  and  are  so  simple  in  structure  that 
it  has  been  impossible  to  relate  them  to  any  other  living  group.  Indeed,  the  ques- 
tion of  whether  they  can  properly  be  called  "living,"  since  they  can  be  crystal- 
lized, has  even  been  raised.  Here,  too,  as  with  the  bacteria,  it  has  been  suggested 
that  they  are  degenerate  rather  than  primitive. 

Bryophyta 

The  so-called  higher  plants  are  now  placed  in  the  subkingdom  Em- 
bryophyta  and  have  the  following  traits  in  common:  terrestrial  plants,  multi- 
cellular embryos  that  are  retained  in  the  female  sex  organs,  and  an  alternation  of 


EVOLUTION    IN    PLANTS  -151 

a  multicellular  gametophyte  generation  with  a  multicellular  sporophyte  genera- 
tion. Both  phyla  in  the  Embryophyta — that  is,  the  Bryophyta  (mosses,  liver- 
worts, and  hornworts)  and  the  Tracheophyta  (vascular  plants) — are  thought  to 
be  descended  from  the  green  algae  (Chlorophyta) .  The  category  Embryophyta, 
like  Thallophyta,  is  an  artificial  one  because  the  bryophytes  and  the  vascular 
plants  appear  to  have  originated  independently  from  the  green  algae.  Although 
it  was  formerly  believed  that  the  bryophytes  gave  rise  to  the  vascular  plants,  the 
first  fossils  of  vascular  plants  come  from  Devonian  and  Silurian  deposits  whereas 
fossil  bryophytes  have  not  been  found  until  millions  of  years  later  in  the  Car- 
boniferous. Thus,  the  present  belief  is  that  the  bryophytes  appear  to  represent  an 
evolutionary  dead  end  because  they  became  adapted,  without  complete  success,  to 
terrestrial  life,  but  have  never  given  rise  to  any  further  better  adapted  groups  of 
plants. 

The  bryophytes  are  small  in  size,  lack  true  roots,  stems,  and  leaves  as 
well  as  vascular  tissue  (xylem  and  phloem),  and  have  a  rather  small  sporophyte 
generation  that  is  dependent  or  parasitic  on  the  larger,  independent  gametophyte 
to  which  it  remains  attached.  They  depend  on  water  for  fertilization,  since  the 
motile  sperm  swim  to  the  egg,  and  in  this  they  can  be  compared  to  the  Amphibia, 
a  group  that  also  has  become  largely  terrestrial  but  in  which  breeding  still  ordi- 
narily must  take, place  in  the  water.  In  fact,  only  the  gymnosperms  and  angio- 
sperms  do  not  require  "environmental"  water  for  fertilization. 

Tracheophyta 

In  contrast  to  the  bryophytes,  the  sporophyte  is  the  predominant  inde- 
pendent generation  in  the  tracheophyte  life  cycle.  The  Tracheophyta  or  vascular 
plants  are  characterized  by  the  presence  of  some  type  of  tracheary  element  and  a 
vascular  system  made  up  of  xylem  and  phloem,  and  all  are  land  plants  except  a 
few  that  have  secondarily  returned  to  water.  In  the  tracheophytes  the  root  system 
is  adapted  for  the  absorption  of  water  and  salts  that  are  transported  to  the  shoot 
system,  which  is  adapted  for  photosynthesis.  The  manufactured  food  is  carried 
throughout  the  plant  by  the  vascular  system.  The  shoot,  exposed  to  the  air,  is 
protected  against  water  loss  by  a  cuticle,  but  openings  or  stomata  permit  the  ex- 
change of  gases  with  the  atmosphere. 

Origin  of  Vascular  Plants 

The  exact  origin  of  the  vascular  plants  is  still  a  mystery,  but  they  are 
now  generally  thought  to  have  been  derived  from  the  green  algae  through  the 
differentiation  of  the  thallus  into  root  and  shoot.  The  discovery  of  a  very  ancient 
order  of  fossil  plants,  the  Psilophytales,  has  tended  to  support  this  theory,  for 
they  are  of  extremely  simple  structure  and  can  be  thought  of  as  a  group,  yet 
various  members  show  indications  of  having  given  rise  separately  to  the  Lycop- 


152  •  THE    EVIDENCE    FOR   EVOLUTION 

sida  (club  mosses  or  ground  pines),  the  Sphenopsida  (horsetails),  and  the 
Pteropsida  (ferns,  conifers,  and  flowering  plants).  See  Fig.  14-2. 

Some  of  the  Psilophytales  resemble  algae  because  they  have  dichotomous 
branching  but  no  leaves  or  roots.  However,  they  differ  in  having  a  cuticle, 
stomata,  a  vascular  system,  and  cutinized  spores.  Furthermore,  certain  psilophytes 
have  very  small  leaves  suggesting  the  club  mosses,  while  others  indicate  leaf 
formation  of  a  different  type,  by  the  flattening  and  broadening  of  the  branch 
system.  In  this  case  the  leaves  are  comparable  to  those  of  broad-leaved  plants 
such  as  the  ferns.  Still  another  type  shows  the  whorled  pattern  characteristic  of 
the  horsetails.  Thus,  within  this  one  group  are  found  fossil  types  suggestive  of 
all  of  the  other  subphyla  of  vascular  plants.  The  subphylum  Psilopsida,  well  rep- 
resented as  fossils  in  the  Silurian  and  Devonian  some  350  to  380  million  years 
ago,  are  now  represented  by  just  two  genera  of  the  order  Psilotales. 

The  Lycopsida  are  another  group  that  appear  to  have  had  their  heyday 
in  the  Paleozoic  and  have  persisted  in  a  few  genera  as  a  relatively  insignificant 
part  of  the  present-day  flora.  In  the  Carboniferous,  the  coal  that  was  formed 
came  from  the  remains  of  these  and  other  plants.  Their  leaves  are  structurally 
simple  and  spirally  arranged,  branching  is  dichotomous,  and  unlike  the  psilopsids 
they  have  distinct  roots,  stems,  and  leaves. 

The  horsetails,  like  the  Lycopsida,  arose  in  the  Devonian,  flourished  in 
the  Carboniferous,  and  have  since  dwindled  into  insignificance.  Perhaps  their 
most  striking  character  is  the  arrangement  of  the  small  leaves  in  whorls,  but  they 
also  have  roots  and  jointed  stems. 

The  dominant  living  plants  belong  to  the  Pteropsida.  Of  these,  the  ferns 
appear  to  be  the  oldest  group  and  are  thought  to  have  given  rise  to  the  seed 
plants.  The  ancient  ferns,  along  with  the  horsetails  and  the  club  mosses,  formed 
the  dominant  vegetation  of  the  Carboniferous.  The  ferns  also  appear  to  have 
evolved  directly  from  the  Psilophytales. 

The  gymnosperms,  to  which  the  conifers  belong,  seem  to  have  evolved 
from  the  ferns  through  the  seed  ferns  (Cycadofilicales),  fossil  seed  plants  with 
many  fern  like  traits.  All  of  the  gymnosperms  are  woody  plants  with  naked 
seeds. 

The  angiosperms  or  flowering  plants,  which  are  dominant  in  the  present 
flora,  present  a  complete  mystery  with  respect  to  their  origin.  They  are  generally 
considered  to  have  evolved  from  one  of  the  groups  of  gymnosperms,  but  even 
though  the  Cycadofilicales,  the  Bennettitales,  the  Gnetales,  and  the  Caytoniales 
have  all  been  suggested  as  progenitors  of  the  angiosperms,  there  is  no  reliable 
evidence  at  present  in  support  of  any  one  of  these  gymnosperm  groups  or  of  any 
other.  The  fossil  record  is  of  little  help,  for  many  fossils  of  flowering  plants  are 
found  in  Cretaceous  deposits,  but  no  older,  possibly  transitional  forms  have  yet 
been  discovered.  Within  the  angiosperms,  it  is  thought  that  the  Ranales  (butter- 
cups and  magnolias)  are  the  most  primitive.  These  plants  belong  to  the  dicoty- 


EVOLUTION    IN    PLANTS  •   153 


<c^v — mr^ 


B 


D 


Fig.  14-2.  Representatives  of  the  primitive  order  of  vascular  plants,  the 
Psilophytales.  A,  Rhynia — simple  member  of  group.  B,  Asteroxylon — possi- 
bly related  to  the  ancestors  of  the  Lycopsida  (the  club  mosses).  C,  Hyenia — 
possibly  related  to  the  ancestors  of  the  Sphenopsida  (the  horse  tails). 
D,  Pseudosporochnus — possibly  related  to  the  ancestors  of  the  Pteropsida 
(the  broad-leafed  plants).    (With  permission  of  Fuller  and  Tippo.) 


154  •  THE    EVIDENCE    FOR   EVOLUTION 

ledons  (mustards,  poppies,  roses,  peas,  composites,  etc.),  which  have  two  seed 
leaves  serving  as  storage  organs  for  food.  The  monocotyledons  (grasses,  lilies, 
palms,  etc.)  used  to  be  considered  more  primitive  but  are  now  thought  to  have 
been  derived  from  the  dicots. 

In  plants  as  in  animals,  many  phylogenetic  questions  remain  to  be 
answered.  Although  it  is  not  unreasonable  to  suppose  that  answers  will  be  found 
to  some — for  example,  the  origin  of  the  angiosperms — on  the  other  hand,  com- 
pletely satisfactory  answers  to  others  may  never  be  forthcoming.  However,  new 
discoveries  continue  to  be  made  and  new  insights  gained,  so  that  in  time  the  rela- 
tionships among  living  things  will  be  much  better  understood  than  they  are  at 
present. 


SUMMARY  < 


The  classification  of  the  plant  kingdom  has  recently  been 
rather  extensively  revised.  This  revision  was  designed  to  bring 
the  system  into  better  accord  with  current  thought  on  phylo- 
genetic relationships  among  plants.  The  general  effect  has  been  to 
separate  the  algae  into  distinct  phyla,  thus  emphasizing  the  differ- 
ences among  them,  while  grouping  the  higher  vascular  plants  into 
a  single  phylum,  Tracheophyta.  Studies  in  paleobotany  as  well  as 
plant  anatomy  are  making  the  history  of  evolution  within  the 
plant  kingdom  increasingly  well  understood.  Although  many  de- 
tails remain  to  be  learned,  the  record,  even  as  it  stands,  is  a  clear- 
cut  case  for  evolution. 

SUGGESTED  READING 

Arnold,  C.  A.,  1947.  An  introduction  to  paleobotany.  New  York:  McGraw-Hill. 

Axelrod,  D.  I.,  I960.  "The  evolution  of  flowering  plants,"  Evolution  after  Darwin. 
Vol.  I,  The  evolution  of  life.  S.  Tax,  ed.  Chicago:  University  of  Chicago 
Press. 

Bold,  H.  C,  1957.  Morphology  of  plants.  New  York:  Harper. 

Fuller,  H.  J.,  and  O.  Tippo,  1954.  College  botany,  2d  ed.  New  York:  Holt. 

Stebbins,  G.  L.,  1950.  Variation  and  evolution  in  plants.  New  York:  Columbia  Uni- 
versity Press. 

Walton,  J.,  1953.  An  introduction  to  the  study  of  fossil  plants.  London:  Black. 


CHAPTER 


15 

Genetic  Evidence 


Hybridization 


A  matched  team  of  mules  is  a  sight  rapidly  passing  from 
the  American  scene.  The  proverbial  stubbornness  and  hardiness 
of  the  mule  are  no  doubt  responsible  for  developing  the  equally 
renowned  vocabulary  of  the  muleskinner.  To  the  question,  "What 
is  a  mule?"  several  answers  can  be  given  in  addition  to  what  a 
muleskinner  might  have  to  say  about  their  character  and  person- 
ality. A  mule  is  a  species  hybrid,  the  offspring  of  a  jackass  (Equus 
asmus)  and  a  mare  (Equus  cabalius),  and  as  such  is  a  prime 
example  of  hybrid  vigor  or  heterosis,  a  phenomenon  frequently 
observed  in  the  progeny  of  two  genetically  dissimilar  individuals 
(see  Fig.  15-1).  A  mule  is  also  an  evolutionary  dead  end,  for 
with  very  rare  exceptions  mules  are  sterile.  By  their  very  existence 
mules  pose  the  question,  "Why  can  two  clearly  distinct  species 
hybridize?"  and  still  another,  "Since  they  can  form  viable,  vigor- 
ous offspring,  why  are  these  offspring  sterile?"  The  answers  to 
the  enigma  of  the  mule  are  wrapped  up  in  the  theory  of  evolu- 
tion. The  hereditary  material  of  the  two  species  is  quite  evidently 
sufficiently  similar  for  fertilization  to  occur  and  for  normal  devel- 
opment to  proceed  under  the  joint  control  of  the  genes  from  both 
species.  The  formation  of  normal  gametes  (or  sperm  and  egg 
cells)  requires,  however,  the  pairing  of  similar  or  homologous 
chromosomes.  Since  the  chromosomes  of  these  two  species  differ 
in  both  number  and  composition,  normal  pairing  or  synapsis  can- 
not take  place.  From  that  point  on,  normal  gamete  formation  is 
disrupted.  The  interpretation  is  that  these  species  trace  back  to  a 


155 


156  •  THE    EVIDENCE    FOR   EVOLUTION 


Equus  ? 

Fig.  15-1.  The  existence  of  the  mule,  sterile  offspring  of  the  cross  between  mare 
and  jack,  is  readily  explained  by  the  theory  of  evolution.  These  two  species, 
descended  from  a  common  equine  ancestry,  are  still  enough  alike  genetically  to 
produce  a  viable  hybrid  when  crossed,  but  their  chromosomal  and  genetic  differ- 
ences are  too  great  to  permit  normal  meiosis  and  gamete  formation  in  the  mule. 


common  ancestor  in  the  not  too  distant  past,  and  that  their  genetic  mate- 
rials are  still  sufficiently  similar  to  permit  normal  fertilization  and  develop- 
ment. However,  during  the  course  of  evolution  their  chromosomes  and  genes 
have  diverged  to  such  a  degree  that  they  no  longer  are  enough  alike  to  permit 
normal  gamete  formation.  Other  theories  leave  unanswered  the  question  of  why 
hybridization  is  possible  at  all  in  two  clearly  distinct  species  such  as  these. 

Man  has  attempted  many  other  crosses  between  different  species,  and 
long  lists  have  been  compiled  of  the  results  of  these  crosses,  many  of  which  have 
been  successful.  In  general,  the  greater  the  similarity  between  the  species,  the 
greater  the  likelihood  of  success  in  hybridizing  them.  Each  successful  cross  raises 
once  again  the  question  of  why  such  crosses  are  possible  if  each  species  had  a 
separate,  independent  origin. 


GENETIC    EVIDENCE  •   157 

In  addition  to  the  artificial  hybrids  many  naturally  occurring  hybrids 
have  also  been  observed,  especially  in  plants.  Relatively  little  work  has  been  done 
with  the  nonvascular  plants — algae,  fungi,  mosses,  etc. — but  in  vascular  plants, 
hybridization  has  been  found  with  unexpected  frequency  in  a  variety  of  different 
groups.  Natural  hybrids  have  been  reported  in  ferns  and  in  a  number  of  genera 
of  conifers  or  gymnosperms  such  as  pine,  juniper,  and  fir.  Among  the  flowering 
plants  or  angiosperms  the  number  of  known  natural  hybrids  continues  to  increase 
as  further  study  brings  to  light  more  and  more  instances  of  hybridization.  Some 
groups  of  woody  plants  such  as  the  oaks  and  willows  seem  especially  likely  to 
form  natural  hybrid  swarms.  Certain  other  groups  have  been  so  disrupted  by 
natural  hybridization  and  its  after-effects  that  their  taxonomy  is  almost  a  hopeless 
mess.  Among  them  are  the  blackberries  (Rubus),  the  hawthorns  (Crataegus) , 
the  dandelions  (Taraxacum),  the  hawkweeds  (Hieracium) ,  and  many  genera  of 
grasses. 

Though  less  common  than  in  plants,  natural  hybrids  in  animals  are  by 
no  means  unknown.  Among  the  invertebrates  only  a  few  phyla  have  been  care- 
fully studied  for  natural  hybrids.  However,  despite  rather  careful  study  in  the 
insects,  relatively  few  natural  hybrids  have  been  reported,  the  best  known  cases 
being  among  the  crickets  and  the  butterflies  and  moths.  Among  the  vertebrates 
quite  a  number  of  natural  hybrids  have  been  reported  in  the  fresh-water  fishes 
such  as  the  sunfish,  suckers,  and  trout.  Hybrid  swarms  of  toads  of  the  genus 
Bufo  are  examples  from  the  amphibians,  and  quite  a  few  hybrids  between  dif- 
ferent species  of  birds,  particularly  the  ducks,  have  been  recorded.  Hybridization 
in  the  reptiles  and  in  the  mammals  is  apparently  quite  rare.  It  seems  probable 
that  ethological  or  "psychological"  isolation,  not  a  factor  in  plants,  contributes  in 
a  significant  way  to  the  rarity  of  natural  hybrids  in  animals.  However,  these  few 
examples  should  suffice  to  show  that  even  without  man's  intervention,  hybridiza- 
tion does  occur  in  both  plants  and  animals.  The  theory  of  evolution  gives  a  rea- 
sonable explanation  for  this  capability. 

Not  only  have  many  casual  or  accidental  hybrids  been  reported,  but 
many  species  of  plants  have  actually  arisen  subsequent  to  hybridization.  Amphi- 
ploidy  (also  called  allopolyploidy;  a  more  detailed  discussion  of  polyploidy  will 
be  given  later)  is  the  result  of  the  doubling  of  the  chromosome  number  of  a 
sterile,  interspecific  hybrid  and  gives  rise  to  a  stable,  fertile,  true-breeding  new 
species  at  a  single  step.  It  is  one  of  the  commonest  ways  in  which  new  species  of 
plants  have  arisen,  and  approximately  a  third  of  the  species  of  flowering  plants 
are  estimated  to  have  originated  in  this  manner.  Many  of  our  most  useful  crops 
such  as  cotton,  wheat,  oats,  tobacco,  and  potatoes  are  now  known  to  be  amphi- 
ploids.  In  the  case  of  wheat,  cotton,  and  tobacco,  good  evidence  as  to  the  actual 
parental  species  is  available.  The  cultivated  tobacco,  Nicotiana  digluta,  was  the 
first  species  to  be  artificially  resynthesized  from  its  parent  species,  N.  tabaccum 
and  N.  glutinosa.  The  first  Linnaean  species  to  be  artificially  recreated  was 


158  •  THE    EVIDENCE    FOR   EVOLUTION 

Galeopsis  tetrahit,  which  was  derived  from  a  hybrid  between  G.  pubescens  and 
G.  speciosa.  Spartina  townsendii  and  two  amphiploids  in  the  goats  beard 
(Tragopogon),  the  latter  two  known  to  have  arisen  in  the  last  30  years,  are 
examples  of  species  that  have  originated  in  nature  in  recent  times  under  human 
observation.  Since  the  discovery  of  the  colchicine  technique  for  doubling 
chromosome  numbers,  a  number  of  experimental  amphiploids  have  been  formed 
that  must,  by  all  the  criteria  commonly  used,  be  regarded  as  new  species.  Some 
forty  years  ago,  Bateson  objected  that  despite  all  the  discussion  about  the  origin 
of  species,  no  one  had  yet  observed  this  event.  Although  the  origin  of  species  by 
polyploidy  may  be  a  special  case,  rather  than  what  Bateson  had  in  mind,  the 
fact  remains  that  man  has  now  observed  the  origin  of  species  in  nature  and  has 
also  synthesized  his  own  new  species. 

Domesticated  Species 

Darwin  opened  his  book  The  Origin  of  Species  with  a  chapter  on 
"Variation  under  domestication"  and  later  summarized  his  studies  in  this  area  in 
the  book  entitled  The  Variation  of  Animals  and  Plants  under  Domestication. 
Domesticated  species  are  still  of  considerable  interest,  for  they  give  us  a  magni- 
fied although  somewhat  distorted  view  of  evolution.  Darwin's  work,  though 
significant  even  today,  was  marred  by  the  lack  of  knowledge  of  the  causes  of 
variations  and  of  their  mode  of  inheritance.  He  recognized,  however,  the  rele- 
vance of  this  type  of  study  to  the  problem  of  the  origin  of  species.  A  more 
sophisticated  discussion  couched  in  modern  genetic  terms  is  now  possible,  but 
the  conclusions  relating  to  the  significance  of  domesticated  species  as  evidence 
for  evolution  are  little  different. 

In  brief,  these  conclusions  are  that  domestic  animals  and  plants  are 
descended  from  wild  species.  In  most  cases  they  appear  to  have  been  derived 
from  a  single  species,  but  some  may  have  originated  from  species  crosses.  The 
numerous  breeds  or  varieties  have  arisen  as  the  result  of  both  conscious  and  un- 
conscious artificial  selection  by  man,  and  also,  it  must  be  added,  by  natural 
selection  operating  in  the  new  environments  provided  by  man.  The  origins  of 
many  domesticated  species  are  obscured  in  the  mists  of  antiquity  or  of  prehistoric 
times.  The  dog,  the  horse,  the  pig,  wheat,  rice,  and  corn — these  and  many  others 
were  domesticated  during  times  for  which  no  historical  records  are  available.  In 
other  cases,  domestication  is  so  recent  that  virtually  a  complete  history  of  the 
process  can  be  given.  For  example,  fox  and  mink  breeding  are  less  than  a  century 
old  yet  already  a  number  of  varieties  have  been  developed,  and  the  fruit  fly, 
Drosophila  melanogaster,  from  which  so  much  of  our  knowledge  of  heredity  has 
been  gained,  also  must  be  included  in  any  list  of  recently  domesticated  species. 
Furthermore,  new  breeds  or  varieties  of  the  older  domesticated  species  continue 
to  be  created,  such  as  the  Santa  Gertrudis  cattle,  the  Minnesota  No.  1,  No.  2,  and 
No.  3  hogs,  and  Thatcher  wheat. 


GENETIC    EVIDENCE  •   159 

The  breeds  of  dogs  range  from  Chihuahuas  and  Schnauzers  to  Great 
Danes  and  St.  Bernards;  of  horses,  from  Shetland  ponies  to  Percherons — yet 
despite  their  great  differences  in  size  and  other  traits,  all  dogs  are  regarded  as 
belonging  to  one  species,  as  are  all  horses.  The  dogs  belong  to  a  single  species 
because  all  the  many  breeds  are  capable  of  hybridizing  except  where  extreme  size 
differences  intervene,  and  even  then  indirect  exchange  of  genes  takes  place 
through  intermediate  breeds.  Since  the  differences  between  some  of  the  breeds 
of  domestic  animals  appear  to  be  much  greater  than  those  between  some  well- 
defined  and  reproductively  isolated  wild  species,  it  may  be  wondered  why  these 
breeds  have  not  become  reproductively  isolated  also.  Although  no  definitive 
answer  can  be  given,  a  guess  may  be  hazarded  that  even  the  oldest  breeds  have 
been  established  but  a  very  short  time,  a  matter  of  a  few  thousand  years  at  most, 
and  that  this  period  has  not  been  long  enough  for  the  numerous  genetic  differ- 
ences leading  to  reproductive  isolation  to  have  accumulated  in  the  separate 
breeds.  In  other  words,  the  differences,  great  as  they  appear  to  be,  may  still  be 
controlled  by  comparatively  few  of  the  many  genes  in  the  species. 

The  significance  of  domesticated  species  as  evidence  for  evolution  lies 
in  the  fact  that  they  show  that  species  have  changed  and  can  be  changed.  The 
numerous  breeds  exemplify  on  a  small  scale  divergence  or  descent  with  modi- 
fication— in  other  words,  evolution. 

Gene  and  Chromosome  Homology 

Another  type  of  genetic  evidence  for  the  relationship  between  species 
is  drawn  from  a  comparison  of  their  chromosomes.  In  every  individual,  a  set  of 
maternal  chromosomes  is  matched  by  a  corresponding  set  from  the  father,  and 
pairing  or  synapsis  only  occurs  between  the  similar  or  homologous  chromosomes 
of  each  set.  Furthermore,  these  maternal  and  paternal  chromosomes  pair  only  in 
a  very  specific  "gene  by  gene"  fashion.  Hence,  if  pairing  occurs  between  the 
maternal  and  paternal  chromosomes  of  a  hybrid  from  a  species  cross,  it  is  a  rea- 
sonable assumption  that  the  paired  regions  are  homologous,  containing  similar 
genetic  material.  The  best  studies  of  this  type  have  been  conducted  with  species 
with  giant  salivary  gland  chromosomes  belonging  to  the  order  Diptera  and  in- 
cluding fruit  flies  (Drosophila) ,  midges  (Chironomus),  mosquitos  {Anopheles), 
and  gnats  (Sczara).  The  large  size  and  banded  structure  of  the  salivary  gland 
chromosomes  permit  the  specific  identification  of  given  regions.  Since  somatic 
pairing  occurs,  the  band  by  band  pairing  of  homologous  regions  can  be  seen  in 
great  detail.  In  hybrids  from  the  cross  between  Drosophila  melano gaster  and  D. 
simulans,  two  morphologically  similar  species,  most  regions  of  the  chromosomes 
can  be  seen  to  be  alike  and  to  be  paired.  Only  a  few  regions  show  differences  in. 
the  banding  structure,  and  these  remain  unpaired.  Furthermore,  genetic  studies 
have  shown  that  there  are  similarities  in  genetic  behavior  in  the  synapsed  regions 
whereas  the  unpaired  regions  differ  in  their  genetic  contents.  In  general,  species 


160  •  THE    EVIDENCE    FOR   EVOLUTION 

less  alike  morphologically  than  these  two  produce  hybrids  that  have  fewer 
homologous  paired  regions.  The  most  obvious  interpretation  of  these  facts  is  that 
during  the  course  of  evolutionary  divergence,  the  chromosomes,  as  well  as  the 
gross  morphology,  have  been  restructured  and  repatterned.  Moreover,  because  of 
the  specificity  of  chromosome  pairing,  chromosomal  homologies  are  even  more 
sensitive  and  reliable  than  anatomical  homologies. 

Above,  in  passing,  we  mentioned  the  similarities  in  genetic  behavior 
between  homologous  chromosome  regions.  This  material  constitutes  still  another 
link  in  the  chain  of  evidence  for  evolution.  In  brief,  it  has  been  possible  to  show 
that  similar  mutant  types  in  different  species  represent  mutations  of  homologous 
genes.  In  some  cases,  these  gene  homologies  have  been  established  by  crossing 
mutant  types  of  each  species  and  obtaining  mutant  hybrid  offspring  in  the  first 
generation.  This  result  would  not  be  obtained  with  nonhomologous  recessive 
mutants  (that  is,  mutants  expressed  only  when  present  in  double  dose),  for  the 
hybrids  would  then  be  normal  or  wild  type  in  appearance.  In  other  cases,  where 
hybridization  is  impossible,  the  evidence  of  necessity  is  less  direct.  However,  the 
demonstration  of  the  homology  of  individual  genes  in  different  species  represents 
one  of  the  most  precise  bits  of  evidence  for  their  common  ancestry  yet  available. 

The  Hereditary  Material 

The  study  of  the  chemical  nature  of  the  chromosomes  from  species 
ranging  from  viruses  and  bacteria  to  higher  plants  and  animals  has  shown  that 
they  are  composed  of  nucleoprotein,  a  combination  of  protein  and  nucleic  acid. 
Nucleic  acids  are  of  two  kinds:  DNA  or  deoxyribonucleic  acid  and  RNA  or 
ribonucleic  acid.  DNA  is  found  in  the  nucleus  of  cells  while  RNA  may  be  found 
in  both  nucleus  and  cytoplasm.  Chemically  very  similar,  both  have  a  backbone  of 
a  long  chain  of  alternate  sugar  and  phosphate  molecules  with  purine  and  pyrimi- 
dine  bases  attached  to  the  sugars  as  side  groups.  The  differences  lie  in  the  sugars, 
deoxyribose  in  DNA  and  ribose  in  RNA,  and  in  one  of  the  four  bases.  Both 
have  the  purines,  adenine  and  guanine,  and  the  pyrimidine,  cytosine,  in  common, 
but  in  DNA  the  other  pyrimidine  base  is  thymine;  in  RNA  it  is  uracil.  All  of 
the  available  evidence  indicates  that  the  nucleic  acids  carry  the  hereditary  blue- 
print from  one  generation  to  the  next.  In  all  but  a  few  cases  (for  example,  some 
plant  viruses)  DNA  is  the  hereditary  material  while  the  RNA  ordinarily  seems 
to  mediate  protein  synthesis. 

One  type  of  evidence  for  the  hereditary  role  of  DNA  comes  from  the 
discovery  that  the  "transforming  principle,"  which  can  produce  inherited  changes 
when  added  to  bacterial  cells,  is  DNA.  Hereditary  changes  in  the  type  of  poly- 
saccharide capsule  in  pneumococci,  for  example,  are  induced  by  DNA  from  a 
related  strain  rather  than  by  its  polysaccharide.  Furthermore,  when  a  bacterial 


GENETIC    EVIDENCE  •   161 

cell  is  infected  by  a  bacterial  virus,  the  DNA  from  the  virus  penetrates  the 
bacterium  and  initiates  virus  reproduction  there,  but  the  protein  coat  of  the  virus 
is  left  outside  of  the  cell. 

DNA  has  been  shown  to  be  composed  of  two  long  strands  coiled  around 
each  other  to  form  a  double  helix  (Fig.  15-2).  The  bases  of  one  strand  pair  very 
precisely  with  the  bases  on  the  other.  In  fact,  adenine  pairs  only  with  thymine, 
and  guanine  only  with  cytosine.  Hence,  the  sequence  of  bases  on  one  strand 
determines  the  sequence  on  the  other,  a  fact  that  appears  related  to  their  power 
of  self -duplication.  It  might  seem  that  DNA,  limited  to  just  four  bases,  a  single, 
simple  type  of  sugar,  and  phosphate  groups,  would  lack  the  complexity  necessary 
to  control  the  great  variety  of  hereditary  traits  in  hundreds  of  thousands  of 


Fig.  15-2.  Watson-Crick  double  helix  model  of  the  DNA  molecule.  S  =  sugar 
(deoxyribose).  P  =  phosphate.  Purine  bases:  A  =  adenine  and  G  =  guanine. 
Pyrimidine  bases:  T  =  thymine  and  C  =  cytosine.  A  always  pairs  with  T,  and 
GwithC. 


species.  However,  the  order  of  the  bases  in  the  DNA  molecule  is  not  regular  or 
repetitive,  and  the  specificity  and  function  of  the  genes  appear  to  be  determined 
by  the  sequence  of  the  bases  along  the  DNA  chain.  In  this  way  an  enormous 
variety  of  specifications  can  be  encoded  or  spelled  out.  The  picture  now  emerging 
is  that  DNA  specificity  is  conferred  on  RNA,  which  moves  into  the  cytoplasm 
where  it  controls  protein  synthesis.  Thus,  the  DNA  code  is  eventually  imprinted 
on  the  enzymes,  the  protein  compounds  that  carry  on  the  bulk  of  the  metabolic 
activities  of  the  cell. 

The  simple  fact  that  the  ultimate  genetic  material  in  nearly  all  species 
can  be  represented  as  variations  on  a  theme  in  a  single  type  of  compound,  DNA, 
makes  evolution  in  all  its  ramifications  more  readily  comprehensible.  This  fact 
points  up  the  fundamental  similarity  among  all  living  things,  and  the  problem 
eventually  will  be  to  discover  how  DNA  patterns  have  changed  in  the  course  of 
time  to  give  rise  to  the  great  diversity  of  living  species. 


162  •  THE    EVIDENCE    FOR   EVOLUTION 

SUMMARY  < 


The  discovery  of  genetic  principles  has  led  not  only  to 
an  understanding  of  the  mechanism  of  evolution  but  also  to 
further  evidence  for  evolution.  Hybridization  between  distinct 
species  has  been  repeatedly  observed  in  plants  and  animals,  and 
in  the  case  of  polyploids  has  led  to  the  formation  of  new  species. 
The  creation  of  new  species  is,  in  itself,  an  insurmountable  argu- 
ment against  a  static-species  concept.  The  development  of  new 
breeds  and  varieties  under  domestication  is  still  further  evidence 
that  species  under  selection  pressure  can  and  do  change.  The  study 
of  the  genetic  material  itself  has  revealed  homologies  between  dif- 
ferent species  at  all  levels  of  organization,  from  chromosomal  re- 
arrangements to  DNA  structure.  Since  DNA  is  the  stuff  of 
heredity,  the  basic  question  in  the  study  of  evolution  is  to  deter- 
mine how  in  the  course  of  time  DNA  patterns  have  changed. 

SUGGESTED  READING 

Darwin,  C,  1868.  The  variation  of  animals  and  plants  under  domestication.  London. 

Davidson,  J.  N.,  1957.  The  biochemistry  of  the  nucleic  acids,  3d  ed.  New  York: 
Wiley. 

McElroy,  W.  D.,  and  B.  Glass,  eds.,  1957.  The  chemical  basis  of  heredity.  Balti- 
more: Johns  Hopkins  Press. 

Miintzing,  A.,  1959.  "Darwin's  views  on  variation  under  domestication  in  the  light 
of  present-day  knowledge,"  Proc.  Amer.  Philosophical  Society,  103:190- 
220. 

Stebbins,  G.  L.,  1959.  "The  role  of  hybridization  in  evolution,"  Proc.  Amer.  Philo- 
sophical Society,  103:231-251. 

White,  M.  J.  D.,  1954.  Animal  cytology  and  evolution,  2d  ed.  New  York:  Cam- 
bridge University  Press. 


PART 


m 


The  Mechanism 

of  Evolution 


The  remainder  of  the  book,  which  is  devoted  to  the 
mechanism  of  evolution,  may  be  regarded  as  a  more  extensive 
genetic  argument  for  evolution  even  though  it  has  not  been  writ- 
ten from  that  point  of  view.  Before  the  mechanism  of  evolution 
is  considered  in  detail,  it  may  be  helpful  to  state,  rather  briefly 
and  without  too  many  qualifications,  the  essential  points  in  the 
current  concept  of  evolution.  The  theoretical  basis  of  modern  evo- 
lutionary theory  was  developed  primarily  by  R.  A.  Fisher,  }.  B.  S. 
Haldane,  and  S.  Wright. 


163 


164  •  THE    MECHANISM    OF   EVOLUTION 

Darwin  believed  that  a  cross  between  two  unlike  individuals  re- 
sulted in  a  blending  of  their  heredity  and  hence  in  a  loss  of  variability. 
Mendel,  however,  demonstrated  that  heredity  is  particulate  in  nature  rather 
than  blending.  Mendel's  results  led  to  the  realization  in  1908  by  Hardy 
and  Weinberg  that  random  mating  in  a  population  where  all  types  are 
equally  favored  does  not  result  in  a  loss  of  variability,  but  that  the  variability 
remains  constant  from  one  generation  to  the  next.  This  concept  has  come  to  be 
known  as  the  Hardy-Weinberg  law. 

If  evolutionary  change  is  to  occur,  new  kinds  of  hereditary  variation 
must  appear.  These  changes  in  the  hereditary  material,  known  as  mutations, 
have  been  shown  to  occur  spontaneously  at  a  very  low  frequency,  which  can  be 
raised  by  various  forms  of  radiant  energy  and  by  some  chemical  substances. 
Mutations  are  essentially  random  within  the  existing  genetic  system,  and  form 
the  raw  material  of  evolution.  The  knowledge  of  mutations,  both  genie  and 
chromosomal,  and  of  the  mutation  process  is  considerably  greater  today  than  it 
was  a  few  decades  ago. 

Natural  selection  determines  the  fate  of  new  mutations  and  of  the  new 
gene  combinations  resulting  from  Mendelian  recombination.  Only  the  adaptively 
favorable  genes  or  combinations  of  genes  will  persist  and  become  incorporated 
into  the  breeding  population. 

Evolution  is  a  phenomenon  occurring  in  populations,  not  in  individuals. 
The  evolving  unit  is  a  breeding  population.  If  the  size  of  the  population  is  small, 
random  loss  or  fixation  of  genes  may  occur,  quite  apart  from  the  operation  of 
natural  selection.  As  a  result  of  this  "genetic  drift,"  and  also  because  of  the 
greater  likelihood  of  inbreeding,  small  populations  are  apt  to  be  more  homo- 
zygous than  large,  and  consequently  less  able  to  adapt  to  changing  environmental 
conditions. 

A  species  may  consist  of  one  large  randomly  mating  population  or, 
more  often,  of  a  number  of  more  or  less  isolated  breeding  populations.  A  single 
large  population  remains  quite  variable  and  evolves  as  a  unit.  If  each  of  a 
number  of  breeding  populations  is  completely  isolated  from  the  others,  evolution 
will  proceed  independently  in  each,  the  resultant  of  the  pressures  of  mutation 
and  selection  and  of  the  random  effects  of  genetic  drift.  Between  the  extremes  of 
complete  isolation  on  the  one  hand  and  random  mating  on  the  other,  all  degrees 
of  partial  isolation  are  possible.  Each  population  will  then  serve  as  an  evolu- 
tionary experiment,  which,  if  successful,  may  spread  its  influence  to  other  popu- 
lations through  the  gene  flow  made  possible  by  migration.  If  gene  flow  is  too 
restricted,  the  more  successful  population  may  supplant  others  as  the  result  of 
intergroup  selection.  Thus,  the  course  of  evolution  may  be  influenced  by  the 
structure  of  the  species  population,  the  way  in  which  it  is  subdivided  into  breed- 
ing populations,  and  the  degree  of  isolation  and  gene  flow  among  them. 


THE    MECHANISM    OF    EVOLUTION  •   165 

The  great  achievement  of  the  population  geneticists  is  that  they  have 
incorporated  the  four  major  factors  causing  gene  frequency  changes  in  popula- 
tions (mutation,  selection,  genetic  drift,  and  migration)  into  a  mathematical 
model  that  permits  the  consideration  of  the  simultaneous  effects  of  all  of  these 
factors.  Even  though  these  factors  are  as  biologically  diverse  as  mutation,  via- 
bility, mating  preferences,  isolation,  differential  fertility  and  fecundity,  and 
migration,  they  have  all  been  evaluated  in  terms  of  their  effects  on  gene  fre- 
quencies. Evolution,  therefore,  is  now  considered  to  be  essentially  a  series  of 
changes  in  the  kinds  or  frequencies  of  genes  in  populations,  or  more  briefly,  a 
shift  in  the  Hardy- Weinberg  equilibrium.  Since  this  is  the  case,  it  is  essential,  if 
we  are  to  understand  the  mechanism  of  evolution,  that  we  gain  some  grasp  of 
the  genetics  of  populations.  But  first,  we  must  understand  the  basic  principles  of 
genetics. 


CHAPTER 


16 

Mendel's  Laws 


Thus  far,  we  have  considered  the  nature  of  the  biological 
world  and  the  theory  that  explains  how  it  has  achieved  its  present 
state — namely,  the  theory  of  evolution.  The  nature  of  the  evi- 
dence in  support  of  the  theory  of  evolution  has  been  reviewed, 
and  some  idea  of  the  evolutionary  changes  that  have  occurred  has 
been  presented.  The  clearer  it  has  become  that  evolution  is  a  fact, 
the  more  urgent  has  become  the  need  to  explain  how  one  species 
can  evolve  into  another,  and  what  forces  operate  to  make  evolu- 
tionary change  possible. 

Darwin's  proposed  mechanism  for  evolution  was  the 
theory  of  natural  selection.  A  major  weakness  of  his  theory,  which 
he  clearly  recognized,  was  his  lack  of  knowledge  about  the  in- 
heritance of  variations.  Darwin  based  his  theory  of  natural  selec- 
tion on  the  differential  survival  and  transmission  of  hereditary 
variations.  Though  Darwin  studied  heredity  and  variation  inten- 
sively, as  others  did  before  and  after  him,  he  failed  to  find  the 
key  to  the  problem.  The  advent  of  the  science  of  genetics  has 
supplied  some  of  the  missing  knowledge,  and  in  the  process  has 
broadened  and  strengthened  the  theory  of  natural  selection. 

The  first  steps  toward  an  understanding  of  heredity  were 
made  by  an  obscure  monk,  Gregor  Mendel,  who  experimented 
with  the  common  garden  pea  in  a  small  monastery  garden.  Alone, 
without  a  research  team  or  even  a  grant  for  a  research  project,  he 
worked  out  with  beautiful  simplicity  and  in  detail  the  funda- 
mental laws  governing  the  transmission  of  characters  from  parent 
to  offspring  in  sexually  reproducing  plants  and  animals.  A  prob- 

166 


mendel's  laws  •  167 

lem  that  had  intrigued  and  puzzled  men  for  centuries  was  solved  by  a 
man  who  had  twice  failed  his  examinations  to  gain  a  teaching  certificate. 
Yet  his  discoveries  were  apparently  neither  understood  nor  appreciated  by 
the  recognized  scientists  of  the  day,  and  their  significance  was  not  realized 
until  1900,  some  35  years  after  the  work  had  been  completed  and 
published.  The  study  of  heredity,  or  genetics,  as  it  came  to  be  called,  is  thus  a 
science  that,  perhaps  more  than  any  other,  belongs  to  the  twentieth  century. 
During  its  brief  career,  it  has  not  only  contributed  to  our  basic  understanding 
of  the  mechanism  of  heredity,  with  ramifications  in  every  area  of  biological 
thought;  it  has  transformed  the  face  of  the  earth  and  added  incalculable  riches 
to  the  resources  of  the  world  through  the  widespread  use  of  new  and  improved 
varieties  of  plants  and  animals  developed  through  genetic  research. 

The  basic  questions  that  Mendel  answered  were  very  simple.  If  a  father 
and  mother  and  their  child  are  seen  together,  the  resemblances  of  the  youngster 
to  his  parents  can  be  readily  observed.  But  all  children  do  not  show  the  same 
degree  of  resemblance  to  each  parent.  Some  appear  to  be  the  "spitting  images" 
of  their  fathers;  others,  of  their  mothers.  Most  show  some  of  the  traits  of  both 
while  some  seem  to  show  little  resemblance  to  either  parent.  This  strange  and 
varying  assortment  of  similarities  and  differences  between  parents  and  offspring 
had  been  the  stumbling  block  to  all  who  had  previously  attempted  to  study 
heredity.  Any  adequate  theory  of  heredity  must  not  only  explain  how  father 
passes  on  his  big  brown  eyes  to  junior,  and  mother  contributes  her  widow's  peak, 
but  also  where  in  the  world  he  got  that  flaming  red  hair,  the  like  of  which  has 
"never"  been  seen  in  either  family.  Genetics,  then,  is  the  study  of  the  way  in 
which  these  resemblances  are  passed  from  one  generation  to  the  next  and  of  the 
mode  of  origin  of  the  variations. 

Careful  examination  and  observation  of  any  group  of  organisms  will 
show  that  each  individual  within  the  group  is  unique  and  clearly  different  from 
all  the  rest.  Hence,  any  attempt  to  study  heredity  in  a  group  is  almost  hope- 
lessly complex  if  an  effort  is  made  to  study  simultaneously  all  of  the  distinguish- 
ing characters  of  each  individual.  It  is  like  trying  to  pitch  a  tent  in  a  tornado — 
impossible  to  keep  track  of  everything  at  once.  Mendel's  success,  in  large  part, 
was  due  to  the  fact  that,  rather  than  trying  to  follow  the  great  multiplicity  of 
characters,  he  sought  to  answer  the  question  of  how  a  single  trait  with  two  well- 
defined  alternative  conditions,  such  as  yellow  or  green  peas,  was  transmitted 
from  generation  to  generation.  In  this  way,  he  reduced  the  problem  to  its 
simplest  terms.  Although  knowledge  of  the  physical  basis  of  heredity  was  virtu- 
ally nonexistent  at  the  time,  Mendel  realized  that  yellow  or  green  seeds  were 
not  transmitted  as  such  from  one  generation  to  the  next,  but  that  somewhere 
within  the  pollen  and  the  ovule  there  were  factors  that  controlled  the  tendency 
to  develop  one  color  or  the  other.  Over  the  narrow  physical  bridge  of  pollen  and 
ovule  in  plants,  sperm  and  egg  in  animals,  must  pass  all  of  the  factors  that 


168  •  THE    MECHANISM    OF   EVOLUTION 

determine  not  only  the  color  of  the  seeds  but  also  that  a  pea  plant  will  never 
become  a  rose  bush;  not  only  the  color  of  junior's  hair  and  eyes  but  also  that 
he  develops  into  a  man  and  not  a  mouse. 

Since  every  individual  is  the  product  of  a  developmental  sequence  con- 
trolled and  influenced  by  both  heredity  and  environment,  the  observed  variations 
may  be  primarily  due  to  heredity,  or  environment,  or  both.  The  old  nature- 
nurture  or  heredity  vs.  environment  controversy  is  virtually  meaningless.  With- 
out heredity,  there  is  no  organism  at  all,  and  it  therefore  must  play  a  role  in  all 
that  an  organism  is  and  does.  Yet  every  organism  develops  in  an  environment  of 
some  sort,  which  is  always  present  and  whose  role  must  always  be  considered  in 
any  assessment  of  the  individual  organism.  However,  all  traits  are  not  equally 
influenced  by  heredity  and  environment,  for  some  are  more  subject  to  environ- 
mental modification  than  others. 

As  Darwin  pointed  out,  only  the  hereditary  variations  are  important  to 
evolution.  We  shall  therefore  not  be  concerned  here  with  environmental  varia- 
tion, although  from  the  experimental  and  practical  standpoint  it  is  always  a 
factor  to  be  reckoned  with.  Our  problem  is  to  account  for  the  inheritance  of 
both  similarities  and  differences.  Actual  traits,  of  course,  are  not  inherited  as 
such.  Your  eyes  are  the  result  of  a  period  of  embryological  development  from 
the  fertilized  egg,  which  has  no  eyes  at  all;  therefore,  they  cannot  be  trans- 
mitted directly.  We  want  to  know  what  is  transmitted  and  how  it  is  transmitted 
from  one  generation  to  the  next. 

Segregation 

Mendel  studied,  in  all,  seven  traits  in  the  garden  pea,  each  with  two 
well-defined  alternative  conditions.  As  in  much  biological  research,  a  good  deal 
of  his  success  can  be  laid  to  his  choice  of  a  suitable  experimental  organism.  The 
pea  was  extensively  cultivated,  and  many  varieties  with  different  hereditary  traits 
were  readily  available.  The  pea  is  normally  self-fertilized,  so  that  the  danger  of 
contamination  by  foreign  pollen  was  negligible,  yet  it  is  fully  fertile  when 
crossed.  Furthermore,  he  kept  accurate  records  of  the  pedigrees  of  each  of  his 
plants,  and  classified  and  counted  all  of  the  progeny  from  his  crosses.  This  arith- 
metic approach  gave  him  more  insight  into  the  hereditary  process  than  was  pos- 
sible for  those  who  merely  classified  without  counting.  Finally,  as  is  also  often 
the  case  in  research,  there  was  an  element  of  luck  involved.  Although  this  is 
getting  ahead  of  the  story  somewhat,  there  are  only  seven  pairs  of  chromosomes 
in  the  pea,  and  each  of  the  traits  Mendel  chose  happened  to  be  controlled  by  a 
different  pair.  If  any  two  traits  had  been  controlled  by  the  same  chromosome 
pair,  the  seemingly  anomalous  results  he  then  would  have  obtained  might  have 
prevented  him  from  breaking  through  to  the  generalizations  known  as  Mendel's 
laws.  The  chance  of  such  a  choice  of  traits  is,  roughly,  only  1  in  200. 


mendel's  laws  •  169 


What  were  the  results  Mendel  obtained  when  he  crossed  two  pure  lines 
differing  in  a  single  trait?  One  of  his  crosses  was  made  between  a  line  that  pro- 
duced only  full,  round  peas  and  another  that  produced  only  wrinkled  peas 
(Fig.  16-1).  From  this  cross,  all  of  the  progeny,  known  as  the  first  filial  or  Fx 
generation,  were  like  the  round  parent.  For  each  of  the  other  characters,  Mendel 


{ 


Round  y  Wrinkled 


Round 


3  Round  :  1  Wrinkled 


All  wrinkled 


3  Round  :  1  Wrinkled 


Fig.  16-1.     Mendel's  results  with  a  monohybrid  cross  involv- 
ing round  and  wrinkled  peas. 

found  that  the  Fj  progeny  from  crosses  between  pure  lines  were  also  all  like  one 
of  the  parents.  He  therefore  called  dominant  those  traits  that  were  expressed  in 
the  F1}  and  recessive  those  traits  not  appearing  in  the  Fx. 

The  Fj  progeny  were  then  self-fertilized  to  produce  the  F2  generation. 
In  the  F2,  a  ratio  of  3  round  plants  to  1  wrinkled  was  obtained.  The  F2  wrinkled 
plants  all  bred  true  for  wrinkled,  but  of  the  F2  round  plants,  one-third  bred  true 
while  two-thirds  behaved  like  the  Fl5  giving  3  round  to  1  wrinkled  offspring. 


170  •  THE    MECHANISM    OF    EVOLUTION 


From  these  results,  Mendel  drew  certain  inferences.  Since  wrinkled  was 
present  in  one  of  the  parents  but  was  not  observed  at  all  in  the  Fl5  some  sort  of 
a  factor  for  it  must  have  been  present  but  not  expressed  in  the  Fx  generation. 
Therefore,  the  ¥r  carried  a  factor  for  wrinkled  as  well  as  for  round,  and  hence 
was  a  hybrid.  Since  the  wrinkled  trait  appeared  unchanged  in  the  F2,  passage  of 
the  factor  for  wrinkled  through  the  F1  hybrid  did  not  affect  its  nature  or  purity. 
See  Fig.  16-2. 


{ 


RR 
Round 


& 


rr 
Wrinkled 


P1   gametes 


{ 


\^/ 


F1  gametes       j  R  :    2  r 


F,    o* 


Rr 
Round 


V  gametes 

1  X           R 

r 

R 

m   m  rr 

$ 

Round 

Round 

r 

•  * 

^  " 

Round 

Wrinkled 

F2  breeding 
behavior  RR 


2Rr 


{ 


RR 


F3l       Round 


3R        :   Irr 
3  Round   :  1  Wrinkled 


Wrinkled 


Fig.  16-2.     Mendel's  interpretation  ot  the  results  trom  the  mono- 
hybrid  cross  with  round  and  wrinkled  peas. 


MENDEL'S    LAWS  •   171 


Furthermore,  the  reappearance  of  the  pure-breeding  wrinkled  and  pure- 
breeding  round  plants  in  the  F2  meant  that  the  factors  for  round  and  wrinkled, 
which  were  present  together  in  the  ¥t  hybrids,  must  have  been  separated  or 
segregated  before  the  formation  of  the  F2.  Therefore,  although  the  ¥x  plants 
were  hybrids,  their  gametes,  or  sex  cells,  must  have  been  pure.  The  gametes  must 
carry  either  the  dominant  round  factor  or  the  recessive  wrinkled  factor,  and  must 
be  of  two.  kinds.  The  3 : 1  ratio  could  easily  be  explained  if  the  two  kinds  of 
gametes  were  produced  in  equal  numbers  and  union  of  the  gametes  at  fertiliza- 
tion occurred  at  random.  These  results  and  conclusions  led  to  the  formulation  of 
what  is  now  known  as  Mendel's  first  law,  the  prijidpIeMf  segregation.  It  can  be 
stated  as  follows:  When  a  hybrid  reproduces,  it  transmits  with  equal  frequency 
either  the  dominant  character  of  one  parent  or  the  recessive  character  of  the 
other,  but  not  both. 

These  concepts  can  be  more  readily  visualized  and  handled  if  they  are 
written  out  in  a  convenient  short  form. 

Let  R  =  factor  for  round 
r  =  factor  for  wrinkled 
Then  a  pure  plant  for  round  would  be  RR,  and  for  wrinkled,  rr.  A  cross  between 
the  two,  known  as  a  monohybrid  cross,  can  be  outlined  as  follows,  where  Px  is 
the  first  parental  generation: 

Pi  RR  (round)     X     rr  (wrinkled) 

\  S 

Pi  gametes      all  R  all  r 

Fi  all  Rr  (round) 

s\ 

Fi  gametes  1/2  R     :     1/2  r 


\    Fief 
\  gam 
Fi  9  \ 
gam    \ 

R 

r 

R 

RR 

round 

Rr 
round 

r 

rR 
round 

rr 
wrinkled 

From  the  checkerboard  used  to  get  the  F2,  it  is  readily  seen  why  a  3 : 1  F2  ratio  is 
obtained,  and  also  why  2  of  the  3  round  individuals  must  be  hybrids. 

At  this  point  it  may  be  well  to  introduce  a  few  more  terms  and  con- 
cepts. A  true-breeding  organism,  such  as  an  RR  round  pea  plant  or  an  rr 
wrinkled  plant,  is  said  to  be  homozygous;  a  hybrid  plant,  such  as  an  Rr  plant, 
which  produces  two  kinds  of  gametes,  is  said  to  be  heterozygous.  The  term 
"factor"  used  by  Mendel  has  been  to  a  large  extent  supplanted  by  the  word 
tfgene."  Learning  genetics  is  much  like  learning  a  new  language,  and  just  to 
show  how  the  jargon  is  used,  the  cross  outlined  above  is  said  to  be  between  a 


172   •  THE    MECHANISM    OF    EVOLUTION 

line  that  is  homozygous  for  the  gene  for  round  and  one  homozygous  for 
wrinkled  to  give  a  heterozygous  round  ¥1.  When  inbred,  the  ¥x  produces  an  F2 
consisting  of  1  homozygous  wrinkled  and  3  round,  of  which  %  are  homozygous 
and  %  heterozygous. 

The  concepts  of  genotype  and  phenotype  are  related  to  each  other  and 
are  fundamental.  The  sum  total  of  all  the  traits  expressed  by  the  individual — 
morphological,  physiological,  psychological,  biochemical,  etc. — is  said  to  com- 
prise his  phenotype.  The  sum  total  of  all  of  the  genes  an  individual  carries, 
received  from  his  parents  and  transmissable  to  his  offspring,  is  said  to  be  his 
genotype.  The  phenotype  is  the  product  of  the  genes  in  the  genotype  acting 
within  a  particular  environment.  The  same  genotype  placed  in  different  environ- 
ments— for  example,  cuttings  from  a  single  plant  reared  under  different  climatic 
conditions — will  give  different  phenotypes.  Yet  the  same  phenotype  may  be 
produced  by  different  genotypes,  as  for  example  the  RR  and  Rr  round  peas. 

Dominance  is  not  a  universal  phenomenon.  The  Rr  peas,  for  example, 
are  as  round  as  the  RR  seeds,  but  microscopic  examination  of  the  starch  grains 
shows  them  to  be  intermediate  in  form  between  those  from  RR  and"  rr  seeds. 
Also,  a  cross  between  a  red  variety  and  a  white  variety  of  zinnias  gives  a  pink 
Fj  hybrid,  and  an  F2  of  1  red,  2  pink,  and  1  white.  Such  examples  can  be  multi- 
plied many  times  to  show  that  all  degrees  of  dominance  exist;  it  may  be  com- 
plete, partial,  or  lacking. 

A  human  trait  inherited  in  accordance  with  the  simple  rules  outlined 
above  is  albinism.  Albinos  in  man  are  characterized  by  a  deficiency  in  pigmenta- 
tion and,  frequently,  eye  defects,  in  addition  to  other  anomalies.  Albinism  is 
due  to  a  recessive  gene  in  the  homozygous  condition.  Although  it  is  a  rare  condi- 
tion, there  are  many  normally  pigmented  people  who  carry  this  gene  in  the 
heterozygous  condition.  A  simple  method  for  determining  what  proportion  are 
carriers  is  to  discover  what  proportion  of  the  marriages  of  albinos  to  unrelated 
normally  pigmented  people  result  in  the  production  of  albino  children.  Such 
matings  are  known  as  "test  crosses,"  since  crosses  to  the  homozygous  recessive 
quickly  reveal  the  genotype  of  the  normally  pigmented  parent.  If  the  normal 
parent  is  homozygous,  all  of  the  children  will  be  pigmented. 

Pi  CC  x  cc 

normally  albino 

pigmented  j 

Pj  gametes  all  C  all  c 


Yx  all  Cc 

normally 
pigmented 
but  carriers 


mendel's  laws  •  173 

If  the  normal  parent  is  a  heterozygous  carrier  of  the  albino  gene,  however,  half 
of  the  children,  on  the  average,  will  be  albino. 

Pi  Cc 

normally 
pigmented 
* 
P1  gametes  i/2  C'  Vl  c 


l/2  Cc  <<.  \l/2  cc 

normally  albinos 

pigmented  carriers 

Such  studies  have  shown  that  although  only  about  1  European  in 
20,000  is  an  albino,  approximately  1  in  70  is  a  heterozygous  carrier  of  the  gene 
for  albinism.  Thus,  the  test  cross,  or  the  back  cross  to  the  recessive,  as  it  is  also 
called,  is  the  most  direct  method  of  ascertaining  the  genotype  of  an  individual 
whose  genotype  is  unknown. 

Independent  Assortment 

After  Mendel  had  established  the  way  in  which  single  traits  were  trans- 
mitted from  generation  to  generation,  his  next  question  became:  What  happens 
if  individuals  differing  in  two  traits  are  crossed  ?  In  one  such  cross,  for  example, 
one  of  the  parents  had  wrinkled  and  yellow  seeds  while  the  other  bred  true  for 
round,  green  seeds.  This  cross  produced  a  uniform  Fl5  all  having  round  and 
yellow  seeds,  these  being  the  dominants.  In  the  F2,  however,  four  phenotypes 
appeared,  two  like  the  original  parents  plus  the  other  two  possible  combinations, 
round  yellow  and  green  wrinkled.  Furthermore,  they  occurred  in  a  definite  ratio 
of  9 : 3 : 3 :  i .  Mendel  inferred  from  these  results  that  the  segregations  of  the 
factors  governing  these  two  traits  were  independent  of  each  other.  The  3:1 
segregation  of  one  factor  pair  (green-yellow)  was  completely  independent  of  the 
3:1  segregation  of  the  other  factor  pair  (round-wrinkled).  The  9:3:3:1  ratio 
then  occurs  because,  of  the  %  of  the  seeds  which  are  round,  %  are  yellow  and 
Vi  green;  of  the  J4  which  are  wrinkled,  %  will  also  be  yellow  and  l/i  green. 
Hence, 

%  X  %  =  %6        round  yellow 

3/4  X  y4  =  %6       round  green 

Vi  X  Va  —  %6       wrinkled  yellow 

Y4  X  1/4  =  Y16       wrinkled  green 

These  results  formed  the  basis  of  Mendel's  second  law,  the  principle  of  inde- 
pendent assortment.  The  law,  stated  briefly,  is  that_the_segregation  of  one  factor 
pair  occurs  independently  of  any  other  factor  pair. 


174  •  THE    MECHANISM    OF    EVOLUTION 


3-0 


YYrr 
Yellow  wrinkled 


yyRR 
Green  round 


P1    gametes 


1 


Yr  yR 

xo" 

YyRr 
Yellow  round 


IVD        lv.  In         1 


F,   gametes      JYR  ■.  \Yr  :   jyR   ■.  \yr 


gametes 

F,   ? 

gametes 


r,< 


YR 

Yr 

yR 

y 

YR 

O 

YYRR 

o 

YYRr 

o 

YyRR 

O 

YyRr 

Yr 

o 

YYRr 

YYrr 

o 

YyRr 

Yyrr 

yR 

o 

YyRR 

o 

YyRr 

• 

yyRR 

• 

yyR/ 

yr 

o 

YyRr 

Yyrr 

• 

0 

yyrr 

9  Yellow  round   :  3  Yellow  wrinkled   :  3  Green  round   :  1  Green  wri 
Fig.  16-3.     A  dihybrid  cross  in  peas. 


nkled 


mendel's  laws  •  175 

This  dihybrid  cross,  as  it  is  called,  can  be  outlined  as  follows: 
Pi  YYrr 


Vx  gametes 


YYrr 
yellow  wrinkled 

I 

Yr 


yyRR 

green  round 

4- 

yR 


Fx  YyRr 

all  yellow  round 

Fi  gametes  l/4  YR:  l/4  Yr:  l/4  yR:  l/4  yr 

Since  the  segregations  are  independent  of  each  other,  all  possible  combinations  of 
the  dominant  and  recessive  genes  are  formed  with  equal  frequency  in  the  Fa 
male  and  female  gametes.  See  Fig.  16-3. 


\   FlCf 
\gam 
Fi  9\ 
gam     \ 

YR 

Yr 

yR 

yr 

YR 

YYRR 

YYRr 

YyRR 

YyRr 

Yr 

YYRr 

YYrr 

YyRr 

Yyrr 

yR 

YyRR 

YyRr 

yyRR 

yyRr 

yr 

YyRr 

Yyrr 

yyRr 

yyrr 

The  checkerboard  should  be  examined  carefully.  The  origin  of  the  four 
phenotypes  and  their  ratio  will  then  be  obvious:  %6  of  the  individuals  have  at 
least  one  dominant  Y  and  one  dominant  R;  %6  are  homozygous  yy  but  carry 
dominant  R;  %6  are  rr  but  carry  dominant  Y;  and  only  y1Q  of  the  plants  are 
homozygous  for  both  recessives.  Furthermore,  though  there  are  only  four  pheno- 
types, they  result  from  nine  distinct  and  different  genotypes,  four  of  which  will 
breed  true.  It  should  be  noted  that  the  same  results  in  the  F2  would  have  been 
obtained  if  the  original  cross  had  been 

Pi  YYRR        X  yyrr 

yellow  round       green  wrinkled 


►  SUMMARY 


Variation  is  the  working  material  of  evolution,  but  not 
all  variations  are  inherited;  only  the  hereditary  variations  are  of 
significance  in  evolution.  Therefore,  the  distinction  between 
phenotype  and  genotype  is  fundamental.  Every  individual  carries 


176  •  THE    MECHANISM    OF    EVOLUTION 

two  complete  sets  of  genes,  one  set  coming  from  the  mother,  the 
other  from  the  father.  Each  gamete  carries  only  one  complete  set 
of  genes.  Mendel  discovered  the  orderly  way  in  which  these  genes 
are  transmitted  from  one  generation  to  the  next.  Each  pair  of 
factors  or  genes  segregates  prior  to  gamete  formation  and  then 
combines  at  random  while  the  different  pairs  of  genes  segregate 
and  recombine  independently  of  one  another.  These  principles 
form  the  genetic  basis  of  variation  through  the  recombination  of 
genes.  Even  though  the  expression  of  some  genes  may  at  times  be 
masked  due  to  dominance,  they  are  not  lost  but  may  reappear  in 
subsequent  generations.  Through  genetic  recombination  an  almost 
infinite  number  of  new  genotypes  can  be  formed  on  which  natural 
selection  can  act. 
"o 
SUGGESTED  READING 

The  birth  of  genetics.  Mendel-de  Vries-Correns-Tschermak.  Supplement  to  Genetics 

35(5),  Part  2. 
Colin,  E.  C,  1956.  Elements  of  genetics,  3d  ed.  New  York:  McGraw-Hill. 
Sinnott,  E.  W.,  L.  C.  Dunn,  and  Th.  Dobzhansky,  1958.  Principles  of  genetics,  5th 

ed.  New  York:   McGraw-Hill.  Appendix  contains  English  translation  of 

Mendel's  original  paper. 
Snyder,  L.  H.  and  P.  R.  David,  1957.  The  principles  of  heredity,  5th  ed.  Boston: 

Heath. 
Srb,  A.  and  R.  D.Owen,  1952.  General  genetics.  San  Francisco:  Freeman. 
Stern,  C,  I960.  Principles  of  human  genetics,  2d  ed.  San  Francisco:  Freeman. 
Waddington,  C.  H.,  1939.  An  introduction  to  modern  genetics.  London:  Allen  and 

Unwin. 


CHAPTER 


17 


Variation  Due  to 

Recombination 


Multiple  Alleles 

Mendelian  inheritance  is  particulate,  the  particulate  genes 
retaining  their  identity  in  crosses.  Segregation  and  recombination 
form  the  basis  of  much  of  the  variability  in  a  species  population. 
Thus  far,  we  have  considered  alternative  forms,  or  alleles,  of  the 
same  gene  to  t>e  of  just  two  kinds,  exemplified  by  the  dominant 
yellow  (F)  and  its  recessive  allele  (j),  or  the  dominant  round 
(R)  and  its  recessive  allele  wrinkled  (r).  Numerous  studies  have 
shown,  however,  that  a  given  gene  can  exist  in  a  number  of  dif- 
ferent alternative  conditions;  hence  a  whole  set  of  alleles  may 
exist  rather  than  only  a  dominant  and  a  recessive.  In  some  cases, 
there  may  be  as  many  as  forty  of  these  multiple  alleles,  as  they  are 
called,  in  a  single  set — that  is,  forty  different  forms  of  the  same 
gene,  each  with  its  own  distinguishable  phenotypic  effects.  How- 
ever, any  diploid  individual  can  carry  in  the  cells  of  his  body  only 
two  of  these  alleles  at  the  most,  while  each  of  his  gametes  can 
carry  but  one. 

Multiple  alleles  open  up  new  ranges  in  the  possibilities 
for  genetic  recombination.  In  the  ABO  blood  groups  in  man,  for 
example,  three  major  alleles  determine  the  blood  types,  the  genes 
being  IA,  lB,  and  1°.  The  blood  types  and  the  genie  combinations 
producing  them  are  as  follows: 


177 


178  •  THE    MECHANISM    OF    EVOLUTION 

blood  type  genotype 
(phenotype) 

O  1°  1° 

A  IAIA  or  IAI° 

B  P1B  or  IBI° 

AB  IAIB 

One  added  allele  increases  the  number  of  possible  genotypes  from  3  to 
6,  and  increases  the  phenotypes  to  4  from  the  two  seen  in  the  F2  of  a  mono- 
hybrid  cross  with  dominance.  Note  that  IA  and  lB  are  both  dominant  to  1°,  but 
not  to  each  other. 

Another  example  may  be  taken  from  the  C  gene  in  the  rabbit.  Four 
alleles  at  this  locus  are  the  following: 

C    —  full  color 
cch  -  Chinchilla 
ch    =  Himalayan 
c     =  albino 

The  C  gene  produces  the  familiar  coat  of  the  wild  rabbit;  cch,  a  pearly  gray  ani- 
mal; ch,  a  white  rabbit  with  black  extremities;  and  c,  a  pure  white  rabbit  with 
pink  eyes.  See  Fig.  17-1. 

With  four  alleles,  10  distinct  genotypes  but  only  four  color  phenotypes 
are  possible,  since  the  dominance  relations  show  C  >  cch  >  ch  >  c.  The  number  of 

different  genotypes  possible  with  n  alleles  can  be  shown  to  equal  .  Hence, 

the  variability  due  to  multiple  alleles  is  by  no  means  trivial  and  increases  very 
rapidly  as  the  number  of  alleles  increases. 

number  of  number  of 

alleles  (n)  possible  genotypes 

1  1 

2  3 

3  6 

4  10 

5  15 

6  21 
10  55 
20  210 
40  820 

These  possibilities  are  restricted  to  just  one  kind  of  gene.  When  it  is  remem- 
bered that  the  total  number  of  genes  in  the  genotype  must  be  in  the  thousands, 
and  that  each  gene  may  have  several  forms,  then  the  number  of  combinations 


VARIATION    DUE    TO    RECOMBINATION  •   179 

possible  among  these  different  sets  of  multiple  alleles  becomes  simply  enormous 
— far  greater  than  the  number  of  individuals  in  the  species.  The  wonder,  per- 
haps, is  not  that  two  individuals  in  a  species  never  look  exactly  alike,  but  that 
they  resemble  each  other  as  much  as  they  do. 

One  further  aspect  of  multiple  allelism  warrants  mention.  Each  of  the 
four  C  genes  has  a  distinctly  different  effect  on  the  phenotype.  Yet  in  specially 
studied  cases,  it  has  been  demonstrated  that  genes  of  different  origin  producing 
the  same  gross  phenotypes,  which  cannot  be  distinguished  from*  one  another  by 


Fig.  17-1.     Variation    in   rabbits   due   to   multiple   alleles.     Top:    left,    full   color; 
right,    chinchilla.     Bottom:    left,    Himalayan;    right,    albino.     (Courtesy   of   Snyder 

and  David.) 


inspection,  nevertheless  have  subtly  different  effects,  either  physiologically  or  in 
their  interaction  with  other  genes  in  the  genotype,  and  hence  must  be  regarded 
as  alleles  rather  than  one  and  the  same  gene.  These  genes  with  equivalent  gross 
phenotypic  effects  that  are  nonetheless  demonstrably  different  are  known  as  iso- 
alleles.  For  example,  in  the  fruit  fly,  a  mutant  type  with  an  interrupted  wing 
vein,  known  as  cubitus  interruptus  fa),  has  been  crossed  to  various  flies  of  dif- 
ferent origin,  all  with  normal  wing  venation,  and  hence  carrying  wild-type  alleles 
of  the  ci  gene.  However,  since  the  expression  of  these  wild-type  genes  in  heter- 
ozygous combination  with  ci  showed  different  degrees  of  effect  on  the  cubitus 
vein,  these  wild-type  genes  are  therefore  isoalleles,  and  were  designated  as  +l5 
+2,  and  +3.  Because  of  the  difficulties  of  detection,  the  amount  of  isoallelism  is 
not  easily  determined,  but  it  is  probably  quite  common,  and  contributes  to  the 
available  variability  in  a  more  subtle  way. 


180  •  THE    MECHANISM    OF    EVOLUTION 

Background  Effects 

Thus  far  we  have  considered  the  gene  to  act  independently  in  producing 
a  trait,  with  a  one-to-one  relation  between  gene  and  character.  Actually,  any  trait 
is  produced  by  the  action  of  many  different  genes  plus  the  effects  of  the  environ- 
ment. Hence,  not  only  the  numbers  of  combinations  of  genes,  but  the  possi- 
bilities for  interaction  between  them  and  between  the  genes  and  the  environment 
must  also  be  considered,  for  genes  do  not  act  in  a  vacuum.  In  the  snapdragon  an 
ivory  variety  (rr)  and  a  red  variety  (RR)  are  known.  The  Fx  hybrid  (Rr),  if 
grown  in  bright  light  at  a  low  temperature,  is  red;  if  grown  in  the  shade  at  a 
high  temperature,  it  is  ivory.  Thus  the  same  genotype  in  different  environments 
gives  different  phenotypes,  and  the  dominance  relations  can  only  be  defined  by 
specifying  the  environmental  conditions.  Brachyury,  a  short-tail  mutation  in  the 
mouse,  behaves  as  a  dominant  in  the  European  house  mouse,  Mus  musculus,  but 
as  a  recessive  in  the  Asiatic  house  mouse,  Mus  bactrianus,  when  the  same  mutant 
male  is  crossed  to  females  of  both  species.  In  this  case  the  same  gene  placed  on 
different  genetic  backgrounds  rather  than  in  different  environments  produces 
different  phenotypes. 

Recombination  and  Interaction 

To  illustrate  the  point  that  the  combined  action  of  many  genes  is  re- 
sponsible for  a  single  trait,  let  us  consider  the  coat  color  in  mink,  Mustela  vison. 
The  rich,  dark  brown  coat  of  the  wild  mink  is  the  product  of  the  genotype,  PP 
Iplp  AlAl  BB  BgBg  BiBi  CC  00  ss  ff  eb  eb  cm  cm.  These  genes  are  known  to 
affect  coat  color  because  mutant  forms  of  each  have  been  discovered;  undoubt- 
edly still  others  will  be  identified  when  mutant  forms  of  them  are  found.  It  is 
one  of  the  peculiarities  of  Mendelian  genetics  that  the  individual  gene  can  be 
identified  only  when  two  alternative  forms  of  the  gene  exist.  Thus,  in  a  sense, 
the  wild-type  gene  is  an  inference  from  the  mutant  allele.  The  mutant  alleles  of 
the  genes  listed  above  are  as  follows : 

genotype  name  genotype  name 


PP 

— Platinum 

c»  cH 

— Albino 

ip  ip 

— Imperial  platinum 

00 

— Goofus 

al  al 

— Aleutian 

S 

— Black  cross 

bb 

— Brown-eyed  pastel 

F 

— Blue  frost 

bR  h 

— Green-eyed  pastel 

Eb 

— Ebony 

bi  bi 

— Imperial  pastel 

Cm 

— Colmira 

Imagine,  if  you  will,  the  possible  color  combinations  that  could  be  produced  by^ 
suitable  crosses.  Some  of  these  combinations  have  already  been  produced,  with 
spectacular  results,  especially  in  the  names  they  have  received. 


VARIATION    DUE    TO   RECOMBINATION  -181 

Ffpp        — Breath  of  spring  platinum 
Ffbb        — Breath  of  spring  pastel 
al  al  ip  ip — Sapphire 
bbpp        — Platinum  blond 

Although  this  particular  type  has  not  been  synthesized,  it  would  be  most  interest- 
ing to  see  an  animal  of  genotype,  bg  bg  oo,  which  should  probably  be  called 
a  green-eyed  goofus. 

When  different  genes  affect  different  traits,  it  is  relatively  simple  to 
predict  the  outcome  of  crosses  involving  these  genes.  However,  when  different 
genes  affect  the  same  trait,  prediction  is  more  difficult  because  of  the  interactions 
between  the  genes.  Even  the  simplest  such  cross,  involving  just  two  gene  pairs, 
can  illustrate  the  complexities.  In  chickens,  for  example,  the  following  results 
have  been  obtained  in  comb  shape  (see  Fig.  17-2)  : 

Px  rose  X  pea 

Fx  walnut 

1 
F2  9  walnut  :  3  rose  :  3  pea  :  1  single 

This  cross  is  obviously  of  the  dihybrid  type  because  a  9:3:3:1  ratio  is  obtained. 
The  relationships  are  shown  below: 

phenotype  genotype 
Walnut  R-  P- 

Rose  R-  pp 

Pea  r r  P— 

Single  rr  pp 

A  somewhat  more  complex  example  of  interaction  can  be  drawn  from 
the  mouse: 

Px  black  X  albino 

1 
Fi  agouti  (wild  type) 

F2  9  agouti  :  3  black  :  4  albino 

The  Fj  agouti  appears  to  be  a  throw-back  to  the  ancestral  wild-type  mouse. 
However,  the  black  and  albino  reappear  in  the  F2,  which  again  suggests  a  two- 
factor  or  dihybrid  cross,  but  with  a  somewhat  aberrant  9:3:4  ratio.  The  explana- 
tion: 

phenotype  genotype 

Agouti  C-  A- 

Black  C-  aa 

Albino  cc  A-,  and  cc  aa 


182   •  THE    MECHANISM    OF    EVOLUTION 


Fig.  17-2.    Variation  in  comb  shape  in  fowl  due  to  the  interactions  be- 
tween  two   pairs   of   alleles.    A,   rose.    B,   pea.     C,   walnut.    D,   single. 
(With  permission  of  Srb  and  Owen.) 


The  difference  from  the  previous  cross  lies  in  the  fact  that  individuals  homo- 
zygous for  cc  have  no  pigment  whatever,  no  matter  what  other  genes  for  pig- 
ment production  may  be  present.  In  this  case,  then,  the  recessive  c  gene  masks 
the  expression  of  both  the  A  and  the  a  genes.  In  a  sense,  this  phenomenon  is 
like  dominance  in  that  one  type  of  gene  suppresses  another,  but  since  it  involves 
different  gene  pairs  rather  than  alleles,  it  has  been  called  epistasis. 

One  last  example  may  serve  to  illustrate  still  another  ratio  and  give 
some  insight  into  the  mechanism  of  action  of  these  genes.  Certain  varieties  of 
white  clover  produce  fairly  high  amounts  of  cyanide  while  others  have  a  low 
cyanide  content.  A  cross  between  two  low-cyanide  varieties  gave  the  following 
results : 


VARIATION    DUE   TO   RECOMBINATION  •   183 

Pi  low  strain  A  X  low  strain  B 

i 
F1  high  in  cyanide 

F2  9  high  :  7  low 

The  chemistry  of  cyanide  production  in  clover  is  fairly  well  understood, 
and  may  be  outlined  as  follows : 

gene  L  gene  H 

4-  4- 

precursor         enzyme  L  substrate  enzyme  H  cyanide 

substance  *    (cyanogenic  ' 

glucoside) 

Thus  strains  A  and  B  are  both  low  but  for  different  reasons.  Strain  A  with  geno- 
type LLhh  lacks  enzyme  H;  B  of  genotype  HHH  cannot  form  enzyme  L.  The 
proof  of  these  statements  comes  from  testing  the  F2  for  cyanide  in  the  manner 
shown  below. 


proportion 

leaf  extract 

leaf  extract 

leaf  extract 

genotyf 

ofF2 

alone 

+  substrate 

+  enzyme  H 

9 

+ 

+ 

+ 

L-H- 

3 

0 

+ 

0 

UH- 

3 

0 

0 

+ 

L-hh 

1 

0 

0 

0 

llhh 

Here  the  nature  of  the  interaction  is  quite  clear.  A  chain  of  synthesis  is 
involved  that,  if  broken  at  any  point,  produces  the  same  phenotype,  low  cyanide 
content.  Each  step  in  the  chain  depends  on  the  preceding  steps.  If  the  phenotypes 
differed  for  each  type  of  interruption — for  example,  if  the  substances  accumu- 
lated at  the  blockage  points  differed  in  color — then  further  genotypes  could  be 
detected  phenotypically. 

This  material  on  recombination  brings  out  one  of  the  main  advantages 
of  sexual  reproduction;  namely,  the  formation  of  gametes  with  a  random  sample 
of  one  allele  from  each  of  the  thousands  of  allelic  pairs  makes  possible  a  vari- 
ability or  plasticity  that  is  impossible  without  sex.  Individuals  reproducing 
asexually  leave  descendants  with  the  same  genotype  as  their  own,  but  with  sexual 
reproduction,  new  gene  combinations  are  always  produced  at  fertilization.  These 
new  genotypes  do  not  simply  involve  new  ways  of  adding  old  traits  together. 
Through  the  interactions  of  the  genes  in  these  combinations,  very  different  new 
types  of  individuals  may  emerge,  some  of  which  may  have  real  advantages  over 
their  parents.  Hence  in  both  natural  evolution  and  controlled  evolution  or  plant 
and  animal  breeding,  segregation,  independent  assortment,  recombination,  and 
interaction  of  genes  provide  a  potent  means  of  progress  toward  better  adapted  or 
more  useful  plants  and  animals. 


184  •  THE    MECHANISM    OF    EVOLUTION 

SUMMARY  < 


Genes  at  a  given  locus  are  not  necessarily  confined  to 
just  two  alternatives,  the  dominant  and  recessive  alleles,  but  may 
consist  of  a  whole  series  of  multiple  alleles.  Though  each  diploid 
individual  will  have,  at  most,  only  two  alleles  and  each  gamete 
only  one,  the  possibilities  for  variability  within  a  population  are 
greatly  extended  by  multiple  alellism.  The  genetic  variation  of  a 
population  is  further  enhanced  and  diversified  by  the  variety  of 
interactions  among  genes  at  different  loci.  These  epistatic  inter- 
actions add  still  another  dimension  to  the  possibilities  for  genetic 
variation  stemming  from  the  recombination  of  genes.  Since  evolu- 
tionary change  is  dependent  upon  the  available  genetic  variability, 
the  variation  arising  from  recombination  plays  a  significant  role 
in  the  evolution  of  sexually  reproducing  species. 

SUGGESTED  READING 

Demerec,  M.,  ed.,  1958.  "Exchange  of  genetic  material:  Mechanisms  and  conse- 
quences," Cold  Spring  Harbor  Symp.  Quant.  Biol.,  Vol.  23.  Long  Island 
Biological  Ass'n. 

See  also  references  at  the  end  of  Chapter  16. 


CHAPTER 


18 


The  Physical  Basis 

of  Evolution 


The  hereditary  mechanism  elucidated  by  Mendel  ac- 
counted for  the  transmission  of  similarities  and  the .  origin  of 
changes  from  one  generation  to  the  next.  Since  evolution  involves 
change  over  successive  generations,  it  obviously  is  related  to  the 
hereditary  mechanism.  In  fact,  the  mechanism  of  heredity  is  the 
mechanism  of  evolution  as  well.  Both  heredity  and  evolution  have 
the  same  physical  basis,  and  it  is  time  now  that  we  consider  the 
physical  basis  of  evolution.  The  factors  of  Mendel  were  merely 
symbols  or  abstractions.  He  had  no  idea  of  where  they  were  or  of 
what  they  were,  but  postulated  their  existence  in  order  to  explain 
his  data. 

In  the  interval  between  the  publication  of  Mendel's  re- 
sults and  their  rediscovery,  the  study  of  cells,  or  cytology,  pro- 
gressed tremendously.  The  cell  theory  had  been  formulated  only 
a  few  decades  before  Mendel's  time,  and  the  cells  were  then  rec- 
ognized as  the  basic  structural  units  in  both  animals  and  plants, 
but  little  was  known  of  the  details  of  their  structure  or  function. 
The  chromosomes  in  the  nucleus  were  not  even  named  until  1888, 
long  after  Mendel's  work.  That  nuclei  came  from  existing  nuclei 
was  only  recognized  about  1875  by  Strasburger.  The  process  by 
which  new  nuclei  are  formed  was  called  mitosis. 

Mitosis 

Mitosis  is  a  continuous  process,  which,  for  descriptive 
purposes,    has   been    divided    into   phases   or   stages   known    as 


185 


186  •  THE    MECHANISM    OF    EVOLUTION 


INTERPHASE 
nucleus 


Early  PROPHASE 
showing  chromatids 


Late  PROPHASE 

Chromosomes  shorten, 

spindle  forms 


METAPHASE 
Chromosomes  line  up 
at  equatorial  plate 


TELOPHASE 
Formation  of 
2  daughter  nuclei 


ANAPHASE 
Chromatids  separate 


Fig.  18-1.     Mitosis  in  nucleus  with  three  pairs  of  chromosomes. 

prophase,  metaphase,  anaphase,  and  telophase.  The  interphase  between  successive 
mitoses  has  been  called  the  resting  stage,  but  a  more  suitable  term  perhaps  is  the 
metabolic  stage.  During  mitosis  each  of  the  chromosomes  in  the  nucleus  under- 
goes a  longitudinal  doubling  to  form  two  chromatids  (see  Fig.  18-1).  The 
chromatids  of  each  chromosome  separate  during  anaphase  and  move  as  chromo- 
somes to  the  opposite  ends  of  the  cell  where  they  form  two  similar  groups  that 


THE    PHYSICAL    BASIS   OF    EVOLUTION  •   187 


PROPHASE  I 

Each  chromosome  splits 

into  2  chromatids  and 

homologous  chromosomes 

pair  (synapsis)  to  form 

tetrads 


PROPHASE  I 

Tetrads  showing 

chiasmata 


ANAPHASE  I 

Homologous  chromosomes 

of  each  pair  separate 


PROPHASE  II 


c,—  *  vC 

ANAPHASE  II 
Chromatids  of  each  dyad  separate 


Four  haploid 

gamete 

nuclei 


Fig.  18-2.     Meiosis  in  gametocyte  with  three  pairs  of  chromosomes. 


^ 


188  •  THE    MECHANISM    OF    EVOLUTION 

then  reconstitute  two  new  daughter  nuclei.  These  nuclei  become  the  centers  of 
two  new  cells  when  a  new  cell  membrane  forms  between  them.  The  chromosome 
material  in  the  new  cells  is  similar  and  is  also  like  that  of  the  original  mother 
cell.  Mitosis  is  thus  a  precise  means  of  self-duplication  of  the  chromosomes,  and 
all  of  the  cells  in  the  body  produced  by  this  process  should  have  the  same 
chromosome  content. 

Life  Cycle  in  Animals 

Each  of  us  was  formed  by  the  fertilization  of  an  egg  or  ovum  by  a 
sperm  cell.  The  egg  carries  a  set  of  chromosomes  from  the  mother;  the  sperm,  a 
similar  set  from  the  father.  The  fertilized  egg  or  zygote  and  all  the  cells  derived 
by  mitosis  from  it  thus  carry  two  sets  of  chromosomes.  If  no  reduction  in  number 
occurred  prior  to  the  next  fertilization,  the  number  of  sets  of  chromosomes 
would  double  in  each  generation.  However,  a  reduction  in  number  does  occur 
during  the  process  of  meiosis  (Fig.  18-2),  which  may  be  regarded  as  a  modifica- 
tion of  mitosis.  Thus  the  gametes,  sperm  and  egg,  carry  a  single  set  of  chromo- 
somes, one  of  each  type,  and  are  said  to  be  In  or  haploid.  The  body  or  somatic 
cells  with  two  sets  or  a  pair  of  each  type  of  chromosome  are  said  to  be  2n  or 
diploid. 

In  the  formation  of  sperm  and  egg  cells  in  animals,  a  process  known  as 
gametogenesis,  nuclear  behavior  is  basically  similar  in  males  and  females  but  in 
other  ways  spermatogenesis  and  oogenesis  differ.  In  the  testis,  stem  cells  known 
as  spermatogonia  divide  mitotically.  Some  of  these  cells  continue  to  function  as 
stem  cells,  while  othe'rs  enlarge  somewhat  to  form  primary  spermatocytes.  The 
first  meiotic  division  of  a  primary  spermatocyte  then  gives  rise  to  two  secondary 
spermatocytes.  With  the  second  meiotic  division,  four  spermatids  are  formed. 
Metamorphosis  of  the  spermatids,  during  which  much  of  the  cytoplasm  is  lost 
and  a  flagellum  or  tail  is  formed,  leads  to  the  formation  of  four  functional 
spermatozoa. 

The  oogonia  in  the  ovary  are  fewer  in  number  than  the  spermatogonia. 
An  oogonium,  through  the  accumulation  of  cytoplasmic  material,  enlarges  greatly 
to  form  a  primary  oocyte.  The  first  meiotic  division  is  equal  with  respect  to  the 
nuclei,  but  the  great  bulk  of  the  cytoplasm  goes  to  one  cell,  and  the  other  nucleus 
with  very  little  cytoplasm  is  pinched  off  as  the  first  polar  body.  The  second 
meiotic  division  is  also  unequal  cytoplasmically,  so  that  an  egg  and  the  second 
polar  body  result.  Thus  oogenesis  gives  rise  to  only  one  functional  egg  cell  even 
though  as  in  spermatogenesis  four  cells  result  from  the  meiotic  divisions.  In 
higher  animals  the  haploid  condition  is  confined  to  the  gametes  themselves. 
There  is  an  alternation  between  haploid  and  diploid  conditions  each  generation, 
but  the  diploid  condition  restored  at  fertilization  prevails  during  virtually  all  of 
the  life  cycle. 


THE    PHYSICAL    BASIS   OF    EVOLUTION  •   189 


fertilization 


Fig.  18-3.     The  life  cycle  of  an  angiosperm  (corn).    (With  permission  of  Wilson 

and  Loomis.) 


Life  Cycle  in  Plants 

Among  higher  plants  an  alternation  of  generations  also  exists  in  the 
life  cycle.  Two  distinct  stages  are  found,  a  diploid  sporophyte  and  a  haploid 
gametophyte.  The  gametophyte  in  mosses  and  ferns  is  quite  prominent,  but  in 
the  flowering  plants  it  consists  of  just  a  few  cells,  and  the  plant  body  is  the 
sporophyte  generation. 

The  meiotic  divisions  occur  during  the  formation  of  haploid  spores  by 
the  sporophyte.  The  spores,  by  a  series  of  mitotic  divisions,  produce  the  haploid 
male  and  female  gametophytes,  which  in  turn  produce  haploid  gametes.  Union 
of  the  gametes  forms  a  zygote  that  then  develops  into  the  diploid  sporophyte. 

In  angiosperms  (see  Fig.  18-3),  the  sporophyte  or  plant  bears  two 
kinds  of  spores,  usually  within  the  same  flower.  The  male  spores  or  microspores 


190  •  THE    MECHANISM    OF    EVOLUTION 

are  formed  in  the  anthers  of  the  flower;  the  female  spores  or  megaspores  develop 
in  the  ovules  of  the  pistil  of  the  flower.  The  stamens  and  pistil  are  surrounded 
by  accessory  flower  parts,  the  petals  and  sepals. 

In  the  anther,  microspore  mother  cells  enlarge  and  undergo  two  meiotic 
divisions  to  form  a  tetrad  of  male  spores.  The  haploid  unicellular  male  spore 
then  undergoes  a  mitotic  division  to  form  a  tube  nucleus  and  a  generative 
nucleus.  This  binucleate  structure,  the  pollen  grain,  is  the  male  gametophyte. 

The  female  spores  form  from  megaspore  mother  cells.  Each  ovule  con- 
tains a  megaspore  mother  cell  that  divides  meiotically  to  form  a  row  of  four 
cells.  Three  of  these  cells  degenerate,  but  the  fourth  enlarges  to  form  a  func- 
tional female  spore.  The  haploid  nucleus  divides  mitotically  to  form  a  two-, 
four-,  and  finally  eight-nucleate  embryo  sac.  Three  nuclei  collect  at  each  end,  and 
one  of  the  cells  at  one  end  becomes  the  egg.  The  mature  embryo  sac  at  this  stage 
is  the  female  gametophyte,  consisting  of  the  egg  nucleus  plus  two  synergid 
nuclei  at  one  end,  two  polar  nuclei  at  the  center,  and  three  antipodals  at  the 
other  end. 

The  pollen  grain,  after  landing  on  the  end  of  the  pistil,  breaks  open, 
and  the  pollen  tube  grows  down  through  the  tissues  of  the  pistil  toward  the 
ovule.  As  the  tube,  containing  both  tube  and  generative  nuclei,  approaches  the 
ovule,  the  generative  nucleus  divides  by  mitosis  to  form  two  sperm  nuclei.  When 
the  pollen  tube  enters  the  embryo  sac,  the  tube  nucleus  disintegrates  and  a 
double  fertilization  occurs.  One  sperm  nucleus  fertilizes  the  egg  to  form  the 
diploid  zygote;  the  other  unites  with  the  two  polar  nuclei  at  the  center  of  the 
embryo  sac  to  form  the  3w  or  triploid  endosperm,  a  tissue  for  food  storage.  The 
zygote  then  develops  into  the  new  diploid  sporophyte  generation. 

Meiosis 

Meiosis,  in  the  simplest  terms,  consists  of  two  nuclear  divisions  during 
which  the  chromosomes  divide  only  once.  Most  of  the  unique  features  in  meiosis 
occur  during  the  prophase  of  the  first  division.  During  this  time,  the  two  mem- 
bers of  each  pair  of  chromosomes  come  to  lie  side  by  side.  Since  by  the  time  of 
this  synapsis  each  chromosome  has  duplicated  into  two  halves  or  chromatids,  a 
tetrad  of  four  chromatids  is  formed.  Exact  reciprocal  exchanges  between  two 
nonsister  chromatids  frequently  occur.  In  this  way  a  portion  of  a  maternal 
chromatid  is  transferred  to  a  paternal  chromatid  and  vice  versa.  These  exchanges 
are  detected  cytologically  as  chiasmata  in  late  prophase. 

At  anaphase  the  homologous  chromosomes  of  each  pair  separate  to 
form  dyads  of  sister  chromatids,  except  in  regions  where  exchanges  have  oc- 
curred. In  these  regions  both  maternal  and  paternal  segments  are  present.  At  the 
second  anaphase  the  centromere  holding  sister  chromatids  together  divides  and 
the  chromatids  of  each  dyad  go  to  opposite  poles,  no  further  duplication  of  the 


THE    PHYSICAL    BASIS    OF    EVOLUTION  •   191 

chromosomes  having  occurred.  Hence  each  chromatid  in  a  tetrad  comes  to  lie  in 
a  different  nucleus.  A  quartet  of  cells  is  formed,  each  cell  with  one  complete  set 
of  chromosomes  rather  than  the  two  present  in  the  original  cell. 

Sex  Determination 

The  precision  observed  in  the  distribution  of  the  chromosomes  at  mitosis 
and  meiosis  suggested  to  the  German  biologist  Weismann  toward  the  close  of 
the  nineteenth  century  that  the  chromosomes  must  in  some  way  be  involved  in 
the  transmission  of  hereditary  characteristics.  The  proof  for  this  idea  came  years 
later,  and  grew  out  of  the  discovery  of  the  way  in  which  sex  is  determined.  For 
centuries  it  was  believed  that  sex  was  determined  by  environmental  forces  acting 
on  the  embryo  during  its  development.  It  would  be  difficult  to  assess  the  abuses 
to  which  mothers  were  subjected  to  ensure  the  production  of  a  child  of  the  de- 
sired sex,  usually  male.  However,  in  the  early  1900's  it  was  discovered  that  males 
had  an  unequal  pair  of  chromosomes  not  observed  in  females.  The  males,  there- 
fore, produced  two  kinds  of  sperm,  one  bearing  a  large  or  X  chromosome  plus 
one  each  of  the  other  chromosome  types,  the  other  bearing  a  small  or  Y  chromo- 
some plus  a  set  of  the  other  chromosomes  known  as  the  autosomes.  Females  were 
found  to  carry  two  X's  and  two  sets  of  autosomes,  and  their  eggs  after  meiosis, 
one  X  and  one  set  of  autosomes.  The  X  and  Y  chromosomes  were  called  sex 
chromosomes  because  fertilization  of  an  X-bearing  egg  by  an  X-bearing  sperm 
produced  a  female  whereas  fertilization  of  an  X-bearing  egg  by  a  Y-type  sperm 
resulted  in  a  male.  Thus  the  cytological  facts  developed  rapidly,  but  independ- 
ently of  the  development  of  knowledge  about  heredity.  Of  course  cytology 
flowered  late  in  the  nineteenth  century  before  genetics  as  a  science  even  had  its 
start,  but  even  after  1900  and  the  rediscovery  of  Mendel's  laws,  the  two  sciences 
pursued  independent  courses. 

Sex  Linkage 

Then,  among  the  many  red-eyed  fruit  flies  in  Thomas  Hunt  Morgan's 
laboratory  at  Columbia,  a  single  white-eyed  male  was  discovered.  When  crossed 
to  red-eyed  females,  all  of  the  Fa  were  red-eyed.  Inbreeding  the  Fx  gave  a  3  red 
to  1  white  ratio  in  the  F2.  This  result  seems  perfectly  normal,  except  for  the  fact 
that  all  of  the  F2  white-eyed  flies  were  males.  This  unusual  result,  it  was  seen, 
could  be  explained  if  the  gene  causing  white  eyes  were  located  on  the  X  chromo- 
some. The  pattern  of  inheritance  then  would  be: 

XX  X    Y 

Pi 

W  W    X        ,i 

red-eyed  female       white-eyed  male 


192  •  THE    MECHANISM    OF    EVOLUTION 


Pi  gametes 
Fi 


WotW 


Ww 


or  Y 


WY 


red-eyed  female         red-eyed  male 


nFi  gam 
9          X 
Fx  gam       N. 

W 

Y 

W 

WW 
red  9 

WY 
redd" 

w 

Ww 
red  9 

wY 

white  cf 

If  this  assumption  is  correct,  it  should  be  possible  to  predict  the  results 
of  the  reciprocal  cross,  white  female  with  red  male,  as  follows : 


Pi  gametes 
Fi 


XX  X    Y 

\w         X        W 
white-eyed  female  red-eyed  male 


WorY 


Ww 
red-eyed  female 


wY 
hite-eyed  male 


\Fi  gam 
9          X 
Fi  gam       \ 

w 

Y 

W 

Ww 
red  9 

WY 
redd1 

w 

WW 

white  9 

wy 
white  cf 

White-eyed  males  and  red-eyed  females  were  expected  in  the  F1?  with  a  1:1  ratio 
of  red  and  white  in  the  F2,  and  this  was  the  result  obtained.  Thus^  it  seemed 
clear  that  the  gene  for  white  eyes  must  be  on  the  X  chromosome,  and  this  un- 
usual type  of  inheritance,  intimately  associated  with  sex,  came  to  be  called  sex 
linked.  It  marked  the  first  step  toward  proving  that  all  of  the  genes  are  located 
on  the  chromosomes,  the  autosomes  as  well  as  the  sex  chromosomes.  Mendel's 
factors,  then,  are  not  mere  abstractions  but  are  physical  entities  borne  by  the 
chromosomes  in  the  nucleus  of  the  cell.  The  chromosomes  are  therefore  the 
physical  basis  of  heredity  and  of  evolution. 

Though  a  great  deal  still  remains  to  be  learned,  the  chromosomes  are 
now   known   to   be   formed   of  nucleoprotein,   a  combination   of  protein   and 


THE    PHYSICAL    BASIS   OF    EVOLUTION  •   193 

deoxyribonucleic  acid,  with  the  latter  in  all  probability  the  vehicle  of  hereditary 
information.  These  small  bodies,  measured  in  thousandths  of  millimeters,  carry 
the  factors  that  in  large  measure  determine  not  only  man's  outward  appearance — 
his  build  and  height,  his  skin,  eye,  and  hair  color — but  also  less  obvious  traits, 
such  as  disease  resistance,  intelligence,  and  personality. 

The  discovery  that  the  genes  were  located  on  the  chromosomes  opened 
up  entire  new  areas  to  exploration  in  the  search  for  knowledge  about  heredity, 
and  also  gave  new  insight  into  the  mechanism  underlying  Mendel's  laws.  The 
separation  of  maternal  from  paternal  chromosomes  at  meiosis  is  the  basis  of 
Mendel's  first  law  of  segregation.  The  random  alignment  of  chromosome  pairs 
at  metaphase  is  the  basis  of  Mendel's  second  law  of  independent  assortment.  In 
other  words,  the  position  on  the  metaphase  plate  of  the  maternal  and  paternal 
chromosomes  of  one  chromosome  pair  is  independent  of  their  position  in  any 
other  pair;  hence  the  gametes  contain  random  combinations  of  maternal  and 
paternal  chromosomes.  As  the  number  of  chromosome  pairs  increases,  the  num- 
ber of  possible  kinds  of  gametes  grows,  the  number  of  kinds  doubling  with  each 
added  pair.  In  man,  for  example,  with  23  pairs  of  chromosomes,  223  different 
combinations  of  maternal  and  paternal  chromosomes  are  possible  in  the  gametes 
of  a  single  individual.  Small  wonder  that  even  brothers  and  sisters  are  never 
alike. 

The  number  of  genes  in  any  species  far  exceeds  the  number  of  chromo- 
some pairs.  Obviously  some  of  the  different  genes  must  reside  on  the  same 
chromosome.  In  such  cases,  Mendel's  law  of  independent  assortment  does  not 
hold,  for  genes  on  the  same  chromosome  tend  to  stay  together  in  crosses,  and 
are  said  to  be  linked.  The  discovery  that  the  genes  are  on  the  chromosomes  was 
the  basis  of  the  third  major  principle  of  heredity,  the  principle  of  linkage.  How- 
ever, this  linkage  is  not  complete,  for  crossing  over  or  recombination  between 
genes  on  the  same  chromosome  sometimes  occurs.  The  chiasmata  formed  in  first 
meiotic  prophase  are  the  visible  evidence  of  the  exchange  of  segments  of  chro- 
matids between  maternal  and  paternal  chromosomes,  which  forms  the  basis  for 
crossing  over.  Hence,  recombinations  occur  within  as  well  as  between  maternal 
and  paternal  chromosomes,  and  the  amount  of  possible  recombination  is  in- 
creased far  beyond  223. 


The  chromosome  mechanism  is  the  physical  basis  not 
only  of  heredity  but  of  evolution.  The  factors  discovered  by 
Mendel  are  located  in  the  chromosomes.  The  behavior  of  the 
chromosomes  is  responsible  for  Mendelian  segregation  and  inde- 
pendent assortment.  However,  genes  on  the  same  chromosome 
tend  to  be  inherited  as  a  linked  group,  occasionally  broken  up  by 


►  SUMMARY 


194  •  THE    MECHANISM    OF   EVOLUTION 

crossing  over.  A  favorable  combination  of  genes  within  a  chromo- 
some tends  to  be  held  together  and  not  broken  up  completely  in 
the  next  generation.  Natural  selection  preserves  favorable  gene 
combinations,  but  could  not  very  well  do  so  if  completely  inde- 
pendent assortment  of  genes  occurred  each  generation.  Hence, 
even  the  organization  of  genes  into  chromosomes  can  be  regarded 
as  adaptive,  a  means  of  preserving  favorable  gene  combinations; 
recombination  and  crossing  over  give  rise  to  variations,  which 
make  possible  adaptations  to  new  or  changing  environmental 
situations. 

SUGGESTED  READING 

Darlington,  C.  D.,  1937.  Recent  advances  in  cytology,  2d  ed.  Philadelphia:  Blak- 
iston. 

Riley,  H.  P.,  1948.  Introduction  to  genetics  and  cytogenetics.  New  York:  Wiley. 

Swanson,  C.  P.,  1957.  Cytology  and  cytogenetics.  Englewood  Cliffs,  N.  J.,  Prentice- 
Hall. 

White,  M.  J.  D.,  1954.  Animal  cytology  and  evolution,  2d  ed.  Cambridge  University 
Press. 


Pa 


Bl_  b£ 

Yl  X  ~bL 

blue  round  red  long 


Px  gam  Bl  bL 


IL 

back 

U_ 

Jl 

cross 

to 

Ti 

blue  long 

red  round 

CHAPTER 


19 


Linkage 


Linkage  and  Crossing  Over 

Even  though  genes  on  the  same  chromosome  tend  to  be 
inherited  as  a  group,  recombination  or  crossing  over  between 
linked  genes  does  occur.  The  mechanism  of  crossing  over  is  a 
reciprocal  exchange  of  segments  between  two  nonsister  chroma- 
tids, which  occurs  in  the  four-strand  tetrad  stage  of  first  meiotic 
prophase,  and  is  observable  cytologically  as  a  chiasma  and  genet- 
ically as  a  recombinant  or  crossover  phenotype.  The  phenomenon 
of  crossing  over  has  made  it  possible  to  map  out  the  relationships 
between  the  genes  on  the  same  chromosome  pair. 

Let  us  first  examine  a  cross  involving  two  pairs  of  linked 
genes.  The  first  work  in  which  linkage  was  recognized  was  carried 
out  by  Bateson  and  Punnett  with  the  sweet  pea  in  1906.  The 
traits  were  long  (L)  versus  round  (/)  pollen  and  purple  or  blue 
(B)  versus  red  (b)  flowers.  Crosses  involving  these  traits  gave 
the  following  results: 


195 


196  •  THE    MECHANISM    OF    EVOLUTION 


\  Fx 

\gam 

Fi       \ 

n 

percent 

gam       \^ 

bl 

phenotype 

obs. 

obs. 

Bl 

Bl 

Jl 

blue 
round 

153 

43.5 

non  CO. 

bL 

bL 

Ti 

red 
long 

155 

44.1 

non  CO. 

BL 

BL 

~bl 

blue 
long 

23 

6.5 

CO. 

bl 

bl 

red 
round 

21 

6.0 

z.or 

bl 

In  this  instance,  instead  of  25  percent  of  the  total  in  each  of  the  F2 
categories  expected  with  independent  assortment,  there  was  a  great  excess  of  the 
original  parental  types  and  a  deficiency  of  the  recombinant  or  crossover  types. 
Rather  than  50  percent  new  types,  only  12.5  percent  crossing  over  occurred.  This 
frequency  of  crossing  over  is  remarkably  constant  between  any  given  pair  of 
gene  loci. 

Linear  Order  of  the  Genes 

Next  let  us  consider  an  example  involving  three  pairs  of  linked  genes. 
Echinus  (ec)  is  a  recessive  mutant  in  the  fruit  fly  (Drosophila  melanogaster) 
causing  rough  eyes;  scute  (sc),  a  recessive  causing  some  bristles  to  be  missing; 
and  crossveinless  (cv)  eliminates  the  crossveins  of  the  wings.  The  wild-type 
genes  for  all  three  mutants  can  be  designated  by  a  plus,  a  convention  that  makes 
the  following  cross  somewhat  easier  to  follow: 


Pi 


+  ec  +        sc  +  cv 
+  e c  +       sc  +  cv 


Pi  gam 

+  e  c  +        sc  +  cv 

Fi 

00+"+  back 
sc  +  cv  cross 

to 

sc  ec  cv 

sc  ec  cv 

cTo71 


non  CO.  gametes  I 


>v     FlCf 
\  gam 
Fi         NT 

9  gam       N. 

sc  ec  cv 

individuals 
observed 

'        +ec  + 

±jc±   ^ 
sc  ec  cv 

810 

sc-\-  cv 

JC+  cv 
sc  ec  cv 

828 

LINKAGE  •   197 


CO.  gametes 


sc  ec  + 

sc  ec-\- 
sc  ec  cv 

62 

+  +CV 

+  +.CP 

sc  ec  cv 

88 

sc+  + 

sc+  + 
sc  ec  cv 

89 

+  ec  cv 

-f-  ec  cv  : 
sc  ec  cv 

103 

+  +  + 

+  +  + 
sc  ec  cv 

0 

sc  ec  cv 

sc  ec  cv 

0 

sc  ec  cv 

total 

1980 

If  each  gene  pair  were  on  a  different  pair  of  chromosomes,  equal  numbers  of 
flies  would  have  been  observed  in  each  of  the  eight  phenotypic  classes.  However, 
independent  assortment  obviously  did  not  occur,  for  the  numbers  range  from 
0  to  828.  The  crossover  percentage  between  two  linked  gene  loci  is  determined 
by  dividing  the  number  of  individuals  showing  recombination  between  these  two 
loci  by  the  total  number  of  individuals  of  all  types  and  multiplying  by  100. 

C  O 

percent  CO.  =  — -  X  100 

fo  -I—  fift  -4-  n  -4-  n 
percent  CO.  between  sc  and  ec  = T7^ X  100  =  7.6  percent 


percent  CO.  between  ec  and  cv  = 
percent  CO.  between  sc  and  cv  = 


1980 
89+103  +  0  +  0 


62  + 


1980 

)S  +  89  +  103 


X  100=  9.7  percent 


1980 


X  100  =  17-3  percent 


Crossover  percentages  between  linked  genes  may  range  anywhere  from  very  close 
to  0  percent  up  to  50  percent,  depending  on  which  two  genes  are  chosen. 
These  crossover  frequencies  not  only  indicate  that  these  genes  are  linked,  but 
they  also  make  it  possible  to  arrange  them  in  a  definite  linear  order.  This  line, 
with  the  genes  marked  off  at  intervals  determined  by  the  crossover  frequencies, 
is  known  as  a  chromosome  map.  From  the  above  data,  the  following  map  can  be 
constructed : 

0  7.6  17.3 


sc 


ec 


-7.6- 


-9.7- 


17.3- 


198  •  THE    MECHANISM    OF    EVOLUTION 

No  individuals  appeared  at  all  in  the  two  double  crossover  classes,  +++ 
and  sc  ec  cv.  If  crossovers  in  the  two  regions  sc-ec  and  ec-cv  were  independent 
events,  the  expected  probability  of  simultaneous  or  double  crossovers  in  these 
regions  would  be  7.6  percent  X  9.7  percent  ==  0.7  percent.  In  other  words, 
about  14  double  crossover  individuals  would  have  been  expected  in  this  cross, 
but  none  was  observed.  Therefore,  it  appears  that  if  one  crossover  occurs,  the 
probability  of  another  crossover  in  adjacent  regions  of  the  same  chromosome  is 
reduced.  This  phenomenon,  known  as  interference,  indicates  that  crossing  over 
must  involve  segments  of  the  chromatids  rather  than  individual  gene  loci.  Inter- 
ference is  complete,  as  in  this  case,  within  a  certain  distance  from  the  first  cross- 
over, and  becomes  progressively  less  the  farther  away  the  second  crossover  is 
from  the  first.  The  proportion  of  expected  double  crossovers  that  actually  occur 
is  called  the  coincidence,  which  thus  serves  as  an  indication  of  the  amount  of 
interference. 

Actually,  the  only  satisfactory  way  to  represent  the  relationships  of 
linked  genes  graphically  is  to  show  the  genes  as  points  on  a  line.  In  numerous 
linkage  tests  made  with  a  variety  of  species,  if  the  crossover  frequencies,  say  for 
three  gene  loci  a,  b,  and  c,  are  ab  and  be,  then  the  frequency  of  ac  is  either  ab 
plus  be,  as  in  the  example  above,  or  ab  minus  be  if  c  lies  between  a  and  b. 
Results  such  as  these  form  the  basis  of  the  fourth  and  final  major  principle  of 
genetics,  the  linear  order  of  the  genes.  Of  the  four  principles,  Mendel  was  re- 
sponsible for  segregation  and  independent  assortment,  and  Morgan  and  his  co- 
workers for  linkage  and  the  linear  order  of  the  genes. 

Extending  these  test  crosses  makes  possible  a  complete  mapping  of  each 
chromosome.  There  are  only  as  many  linkage  groups  as  there  are  chromosome 
pairs,  and  each  gene  can  be  located  with  respect  to  all  of  the  others.  The  greater 
the  physical  distance  between  two  genes  on  the  same  chromosome,  the  greater 
the  chance  of  recombination  between  them,  and  the  farther  apart  they  will  ap- 
pear on  the  map. 


SUMMARY  <■ 


The  genes,  the  basic  units  of  evolution,  are  located  on 
the  chromosomes  and  are  arranged  in  a  linear  order  that  can  be 
mapped  with  considerable  precision.  Evolution,  therefore,  occurs 
within  the  limits  imposed  by  the  chromosome  mechanism  of 
heredity. 


SUGGESTED  READING 

See  references  at  the  end  of  Chapter  16. 


CHAPTER 


20 

Chromosomal  Variation 


Linkage  studies  and  chromosome  mapping  are  possible 
because  the  structure  of  the  chromosomes  is  very  stable.  On  rare 
occasions,  however,  chromosome  rearrangements  may  occur.  These 
rearrangements  can  usually  be  detected  both  cytologically  and 
genetically,  for  the  linkage  relationships  of  the  genes  are  changed 
by  any  restructuring  of  the  chromosomes.  In  order  for  rearrange- 
ments to  occur,  the  chromosomes  must  break.  Chromosome  break- 
age may  be  "spontaneous,"  but  it  can  also  be  induced  by  such 
agents  as  ionizing  radiation  and  certain  chemical  compounds.  In 
many  cases  the  breaks  heal  or  restitute  with  no  detectable  cytolog- 
ical  or  genetic  effect.  However,  if  the  broken  ends  fail  to  unite  or 
else  reunite  in  new  combinations,  they  then  can  be  detected. 

Duplication  and  Deficiency 

A  number  of  types  of  rearrangements  have  been  recog- 
nized (see  Fig.  20-1).  A  deficiency  or  deletion  may  arise  as 
follows : 

ABC      DEFGH    -4    ABC      DE+FGH 

• • 

T 

breakage  deficiency  for  acentric 

point  FGH  region  fragment 

A  deficiency  is  often  lethal  when  homozygous,  or  even,  if  large 
enough,  when  heterozygous,  and  is  therefore  not  apt  to  play  a 
role  in  evolution. 


199 


200  •  THE    MECHANISM    OF    EVOLUTION 


DELETION 
of  c  -  d  region 


8 


In  synapsis 


INVERSION 
1        of  c-d  region 


DUPLICATION 
of  c  -  d  portion 


synapsis 


In  synapsis 


TRANSLOCATION 
between  nonhomologous 
chromosomes 


n  synapsis 


Fig.  20-1.     Types  of  chromosome  rearrangements. 


way: 


CHROMOSOMAL    VARIATION   •  201 
A  duplication  of  a  chromosome  segment  may  arise  in  the  following 


ABC       DEFGH-*    ABC       DEFEFGH 

• • 


ABC      DEFGH 

• 

T      T 


duplication  for 
EF  region 


The  addition  of  the  extra  EF  segment  gives  rise  to  a  duplication  or  repeat  of  that 
region.  Duplications  are  generally  viable  and  represent  a  way  of  adding  addi- 
tional gene  loci  to  the  genotype.  Furthermore,  it  has  been  suggested  that  muta- 
tion can  then  produce  genes  of  divergent  function  as  follows : 


duplication 


E 


mutation 


E' 


In  this  way,  during  the  course  of  evolution  the  total  number  of  genes  could  be 
increased  with  a  corresponding  diversity  of  function. 

Inversion 

An  inversion  results  when  two  breaks  in  a  chromosome  rejoin  after  the 
fragment  has  rotated  180  degrees. 

ABC      DEFGH    -»    ABC      DGFEH 
\  n  i  inversion 

The  linkage  relations  are  changed  with  G,  for  example,  now  closely  linked  with 
D  rather  than  H.  Inversions  that  include  the  centromere  are  pericentric;  those 
not  including  the  centromere  are  paracentric.  Individuals  may  be  either  homo- 
zygous or  heterozygous  for  an  inversion.  In  inversion  heterozygotes,  the  synapsis 
of  homologous  chromosomes  at  meiosis  is  somewhat  abnormal,  for  homologous 
genes  continue  to  pair  wherever  possible  despite  their  different  linkage  relations 
in  the  two  homologues.  As  a  result  of  these  pairing  forces  the  chromosomes  are 
thrown  into  easily  recognized,  characteristic  loops.  If  pairing  and  crossing  over 
do  occur,  abnormal  chromosomes  and  fragments  are  frequently  produced  that 
are  usually  unviable.  Hence,  the  inversions  act  essentially  as  crossover  suppressors, 
preventing  recombination  within  chromosomes  since  the  crossover  products  give 


202  •  THE    MECHANISM    OF    EVOLUTION 

rise  to  gametes  with  aberrant  haploid  sets  of  chromosomes  for  the  most  part. 
Thus  in  an  evolutionary  sense  inversions  are  conservative  because  ordinarily  only 
the  old  gene  combinations  give  rise  to  viable  organisms. 


Translocation 

A  reciprocal  translocation  arises  when  breaks  in  two  chromosomes  are 
followed  by  reunion  with  the  fragments  interchanged. 

ABC       D  E  N  M 

• 


HGFOPQ      RST 

— •—  ■ • 

T 

Genes  in  the  exchanged  fragments  now  belong  to  new  linkage  groups,  but  the 
genes  will  still  pair  with  their  old  allelic  partners  so  that  in  a  translocation 
heterozygote  four  chromosomes  will  form  a  single  synaptic  figure. 


ABC 

D  E  F  G 

H 

• 
M  N  O  P 

T 

Q      RST 

• 

H 


IV 


ABC 

D  E 

a 

ABC 

D  E 

III 

N 
M 

H 


O  P 

Q 

RST 

* 

O  P 

Q 

RST 

N 
M 

II 

A  little  study  will  show  that  if  chromosomes  I  and  IV  go  to  the  same 
pole,  the  gametes  will  be  deficient  for  the  genes  in  the  region  MN  while  those 
in  region  FGH  will  be  duplicated.  The  reverse  is  true  if  chromosomes  II  and  III 
go  to  the  same  pole.  Because  of  the  deficiencies,  sterility  will  ensue.  Only  combi- 
nations of  I  and  III  or  II  and  IV  can  be  expected  to  be  fertile.  Furthermore, 
crossing  over  may  lead  to  additional  sterility.  If  several  translocations  are  present, 
rings  of  chromosomes,  chains  of  chromosomes,  or  other  unusual  synaptic  con- 
figurations will  be  observed  in  meiotic  prophase  because  of  the  specificity  of  the 
pairing  reaction.  Crosses  between  populations  having  different  gene  arrange- 
ments, whether  inversions  or  translocations,  will  not  ordinarily  be  selectively 
advantageous  since  there  is  partial  sterility  in  the  resulting  progeny.  In  some 
species,  however,  inversions  (for  example,  Drosophila)  and  translocations  (for 


CHROMOSOMAL   VARIATION  •  203 

example,  Oenothera)  have  become  a  part  of  the  normal  genetic  system  within 
breeding  populations,  apparently  having  an  adaptive  function. 


Position  Effect  and  Pseudoallelism 

In  addition  to  changing  the  linkage  relationships,  in  some  cases  re- 
arranging the  relationships  of  the  genes  to  each  other  changes  their  effects  on  the 
phenotype  though  the  genes  themselves  are  apparently  unchanged.  This  phe- 
nomenon is  known  as  position  effect.  The  classical  example  of  position  effect 
involves  Bar  eye  in  the  fruit  fly.  The  Bar-eye  condition  is  due  to  the  duplication 
of  a  small  segment  of  the  chromosome  and  can  be  diagramed  as  follows: 


1.  wild  type 


2.  Bar  eye 


3.  double  Bar 


[] 


4.  double  Bar/ 
wild  heterozygote 


The  genie  contents  of  types  2  and  4  are  identical,  but  the  heterozygote 
has  significantly  smaller  eyes  than  the  homozygous  type.  Hence,  the  phenotypic 
difference  must  be  due  to  the  genes'  arrangement,  and  the  expression  of  a  gene 
is  dependent  not  only  on  its  intrinsic  effects  but  also  on  its  position  with  respect 
to  the  other  genes  in  the  genotype. 

Position  effect  has  also  been  found  to  be  the  rule  with  pseudoalleles. 
The  term  pseudoallele  was  coined  to  describe  cases  originally  thought  to  involve 
a  single  locus  with  multiple  alleles  but  that  turned  out  to  be  two  or  more  very 
closely  linked  loci  with  all  the  genes  affecting  the  same  trait.  One  interpretation 
currently  favored  is  that  these  loci  arose  by  duplication  (hence  their  similarity  in 
action)  followed  by  mutation  to  divergent  functions  as  suggested  above.  The 
white-apricot  case  in  Drosophila  melanogaster  will  serve  as  an  example  of  posi- 
tion pseudoallelism.  The  eye-color  mutants,  white  and  apricot,  were  originally 
thought  to  be  members  of  a  multiple  allelic  series  at  the  white  locus,  and  were 
designated  w  and  w&.  The  discovery  of  rare  crossovers  (approximately  0.01  per- 
cent) between  white  and  apricot  indicated  that  separate  closely  linked  loci  were 
involved,  and  the  mutants  were  designated  w  and  apr.  Position  effect  was  re- 
vealed when  the  phenotypes  of  the  two  kinds  of  double  heterozygotes  were 


204  •  THE    MECHANISM    OF    EVOLUTION 

compared.  In  the  cis  condition  both  mutant  genes  are  on  one  chromosome,  both 
wild-type  genes  on  the  other.  The  trans  state  has  one  mutant  and  one  wild-type 
gene  on  each  homologue. 

apr      w  apr      + 


+       +  +        w 

cis  trans 

The  cis  phase  has  phenotypically  wild-type  red  eyes  whereas  the  trans  has  a 
light  apricot  eye  color.  Since  both  types  of  double  heterozygotes  have  exactly  the 
same  genes,  position  effect  is  obviously  involved.  The  discovery  of  position  effect 
and  of  pseudoallelism  has  led  to  a  considerable  revision  in  the  gene  concept. 

Heteroploidy 

Let  us  now  consider  chromosomal  variations  involving  changes  in  the 
numbers  of  whole  chromosomes  rather  than  rearrangements  involving  chromo- 
some fragments.  Two  general  types  of  change  have  been  found.  Polyploids  (or 
euploids)  are  individuals  with  one  or  more  complete  haploid  sets  of  chromo- 
somes added  to  the  usual  diploid  number.  Heteroploids  (or  aneuploids)  have 
some  number  of  chromosomes  other  than  an  exact  multiple  of  the  haploid 
number. 

A  heteroploid,  for  example,  may  have  an  extra  chromosome  from  one 
pair,  or  In  +  1  chromosomes,  and  is  then  known  as  a  simple  trisomic.  If  a 
chromosome  from  one  pair  is  lacking  (2n  —  1),  it  is  known  as  a  simple 
monosomic.  These  and  more  complex  heteroploids  tend  to  lead  to  sterility  or 
deficient  gametes,  and  hence  are  generally  of  little  evolutionary  significance. 

Polyploidy 

The  changes  involving  whole  haploid  sets  of  chromosomes,  however, 
have  been  of  considerable  evolutionary  significance,  especially  in  plants.  These 
polyploids  may  be  of  several  kinds,  among  the  more  common  being  triploids 
(3«),  tetraploids  (4/z),  hexaploids  (6n),  and  octoploids  (8;?).  Many  domesti- 
cated plant  species  are  polyploid  (wheat,  cotton,  apples,  etc.),  and  it  is  now 
possible  for  plant  breeders  to  induce  polyploidy  with  colchicine,  a  chemical  sub- 
stance that  inhibits  the  formation  of  the  mitotic  spindle.  The  polyploids  fre- 
quently have  more  vigorous  vegetative  growth  and  larger  and  more  intensely 
colored  flowers,  and  hence  are  especially  desirable  as  new  horticultural  varieties. 

Polyploidy  arises  in  two  distinctly  different  ways.  A  multiplication  of  the 
chromosome  sets  from  a  single  species  gives  rise  to  autopolyploidy.  If  A,  for 
instance,  represents  a  single  haploid  set  of  chromosomes,  the  diploid  will  be  AA, 


CHROMOSOMAL   VARIATION   •  205 

and  an  autotetraploid,  AAAA.  Though  vegetative  vigor  is  usually  good,  sterility 
is  high  in  autopolyploids  due  to  abnormal  synapsis  at  meiosis  when  more  than 
two  homologous  chromosomes  form  a  synaptic  figure. 

Allopolyploids  or  amphiploids  are  formed  when  hybridization  between 
two  different  species  is  followed  by  a  doubling  of  the  chromosome  number  in  the 
diploid  hybrid  or  by  the  formation  of  unreduced  gametes: 


p. 

AA                X                BB 

Pt  gametes 

A                              B 

Fx 

AB 

chromosome  doubling 

Fx  gametes 

AB           X           AB 

F2 

AA  BB            allotetraploid 

The  Fi  AB  hybrid  is  generally  quite  sterile  due  to  the  lack  of  pairing 
between  the  chromosomes  of  the  A  and  B  genomes.  The  F2  allotetraploid,  on 
the  other  hand,  is  fertile,  acting  as  a  functional  diploid,  since  each  type  of  A 
and  B  chromosome  is  represented  twice,  and  pairing  at  meiosis  is  normal  be- 
tween these  homologues.  In  some  cases  polyploids  more  or  less  intermediate  to 
the  auto-  and  allopolyploids  have  been  formed,  which  are  known  as  segmental 
allopolyploids. 

More  than  one  third  of  all  species  of  higher  plants,  the  angiosperms, 
are  polyploid,  and  thus  polyploidy  has  been  of  considerable  importance  to  plant 
evolution.  With  the  discovery  of  means  of  inducing  polyploidy,  new  horizons 
have  been  opened  to  the  plant  breeders.  An  early  and  classical  example  of  a 
synthetic  allotetraploid  was  Rap  ban  o  bras ska,  formed  from  the  radish  {Raphanus) 
and  the  cabbage  (Brassica).  Such  a  plant  obviously  had  considerable  potential 
since  the  edible  portions  are  the  root  in  one  parent,  the  shoot  in  the  other. 
Briefly,  the  details  of  the  cross  are  as  follows: 

p. 


radish                  X 

cabbage 

2;?x  =  18 

2n2  -  18 

nx  -  9 

n2  =  9 

»1    +    »2    = 

=  18 

Pi  gametes 

sterile  diploid 

chromosome  doubling 
¥1  gametes  («!  +  n2)  X  (n1  +  n2) 

F2  nx  nx  n2  n2 

fertile  allotetraploid 

One  difficulty  emerged  when  these  sturdy,  fertile  F2  plants  were  examined;  they 
had  a  root  like  a  cabbage  and  a  head  like  a  radish. 


206  •  THE    MECHANISM    OF   EVOLUTION 

SUMMARY  A 


Chromosomal  variation  as  well  as  genie  variation  can  be 
observed  in  natural  populations.  These  variations  include  re- 
arrangements involving  chromosome  fragments  such  as  duplica- 
tions and  deficiencies,  inversions,  and  translocations.  The  addition 
or  loss  of  whole  chromosomes  gives  rise  to  heteroploidy,  in  which 
the  number  of  chromosomes  does  not  equal  an  exact  multiple  of 
the  haploid  number.  Polyploids,  with  additional  complete  haploid 
sets  of  chromosomes,  may  arise  within  a  single  species  or  subse- 
quent to  hybridization  between  different  species.  Chromosomal 
rearrangements  may,  on  occasion,  lead  to  position  effects  when  the 
gene,  in  a  new  location  with  respect  to  the  rest  of  the  genes,  has 
a  changed  effect  on  the  phenotype  even  though  the  gene  itself  is 
apparently  unchanged. 


SUGGESTED  READING 

See  references  at  the  end  of  Chapter  18. 


CHAPTER 


21 


Mutation 


Over  a  century  ago  a  short-legged  ram  unlike  any  of  the 
other  sheep  was  born  into  the  flock  of  a  New  England  farmer 
named  Seth  Wright.  This  ram  transmitted  the  short  legs  to  his 
progeny,  and  from  him  was  thus  derived  the  Ancon  breed  of 
sheep  (see  Fig.  21-1),  valued  by  New  Englanders  because  these 
sheep  were  unable  to  jump  the  stone  fences  so  common  there. 
Apparently  they  were  not  prized  for  very  long,  since  the  breed 
became  extinct  about  eighty  years  ago.  However,  more  recently  a 
Norwegian  lamb  with  short  legs  appeared,  and  from  this  animal 
a  new  strain  has  been  developed.  The  s^ddejn^iLp^aianjce^oi-a 
new  hereditary  trait  in  a  population  is  said  to  be  due  to  a  muta- 
tion^a  change  in  the  hereditary jnaterial.  In  this  case,  the  trajJL 
behaved  as  a  simple  recessive  in  crosses,  and  presumably  had 
its  ^origin  by  mutation  and^  not  by  the  recombination  of  existing 
genes.  A  great  variety  of  mutations  has  been  observed  in  a  number 
of  different  species.  The  valuable  platinum  mutation  in  the  fox, 
streptomycin  resistance  in  bacteria,  and  the  hemophilia  mutation 
("bleeder's  disease")  that  Queen  Victoria  bestowed  so  liberally 
among  her  descendants  are  cases  in  point. 

Types  of  Mutations 

In  a  broad  sense  a  mutation  is  any  hereditary  change  not 
due  to  the  simple  recombination  of  genes.  Included  in  this  sense 
are  gene  or  point  mutations,  chromosomal  changes,  either  struc- 
tural or  numerical,  and  position  effects.  In  a  narrower  sense,  muta- 


207 


208  •  THE    MECHANISM    OF    EVOLUTION 

tion  is  used  to  refer  to  a  self-duplicating  change  at  a  single  gene  locus.  Gene 
mutations  are  of  fundamental  importance  to  evolution  because  they  form  the  raw 
material  of  evolution.  Only  by  mutation  can  truly  new  kinds  of  genetic  variation 
appear,  and  all  evolutionary  change  is  based,  ultimately,  on  mutation.  Mutation 
alone,  however,  cannot  account  for  evolution,  for  the  sporadic  mutants  must  in 
some  way  become  a  part  of  the  genotype  of  the  population. 

There  is  no  simple  method  of  classifying  mutations,  for  they  may  affect 
all  kinds  of  traits  in  the  organism,  from  its  pigmentation  to  its  psychoses,  and 


Fig.  21-1.     Normal  ewe  on  left.    Short-legged  Ancon  ewe  in  the  center  and  ram 
on  the  right  are  homozygous  for  the  recessive  Ancon  mutation. 


they  are  therefore  of  an  almost  bewildering  variety.  One  method  of  classification 
frequently  used  takes  only  the  effect  on  viability  into  consideration,  and  the 
mutants  are  then  classified  as  lethal,  semilethal,  subvital,  normal,  and  supervital. 
Another  common  approach  is  to  group  the  mutations  according  to  their  visible 
effects  on  the  phenotype,  and  mutants  are  described  as  wing  mutants,  eye-color 
mutants,  body-color  mutants,  bristle  mutants,  etc.  However,  the  so-called  "white- 
eye"  mutant  in  the  fruit  fly  also  causes  transparency  of  the  testicular  envelope,  a 
change  in  spermatheca  shape,  and  a  lowered  viability,  longevity,  and  fertility. 


MUTATION  •  209 

Hence,  to  call  white  an  eye-color  mutant  scarcely  indicates  the  entire  story.  These 
genes  with  a  multiplicity  of  effects  are  said  to  be  pleiotropic,  but  the  apparent 
variety  of  effects  may  be  traceable  to  a  single  primary  change  in  gene  function. 
The  observed  phenotypic  effects  are  generally  far  removed  from  the  primary 
action  of  the  gene.  The  biochemical  mutants  in  microorganisms  may  be  some- 
what closer  to  the  primary  gene  action.  These  mutant  types  usually  fail  to  form 
a  particular  biochemical  substance  such  as  an  amino  acid  or  a  vitamin  because  of 
the  absence  or  inactivation  of  an  enzyme  needed  to  mediate  the  synthesis.  Study 
of  mutants  of  this  type  may  in  time  do  away  with  the  need  for  the  more  or  less 
arbitrary  classifications  of  mutants  currently  in  use. 

Induced  Mutation 

"Spontaneous"  mutations  occur  all  the  time,  but  they  are  called  "spon- 
taneous" simply  because  the  exact  causes  are  not  as  yet  well  understood.  The 
mutation  rate  can  be  raised  well  above  this  "spontaneous"  rate  by  various  experi- 
mental techniques  that  have  provided  some  insight  into  the  mechanisms  of 
mutation.  Temperature  shocks  were  one  of  the  first  methods  used  to  raise  the 
mutation  rates;  in  flies,  exposures  for  short  periods  to  both  low  and  high  tem- 
perature extremes  outside  the  normal  range  were  found  to  be  effective.  Within 
the  normal  temperature  range  of  the  organism,  mutation  rates  will  be  higher  at 
the  higher  temperatures. 

The  discovery  that  x-rays  and  other  ionizing  radiations  (a,  f3,  and  y 
rays,  protons,  neutrons)  induced  mutations  and  caused  chromosome  breakage 
marked  a  milestone  in  the  study  of  mutation.  The  number  of  mutations  is  directly 
proportional  to  the  dose  of  radiation  and  is  independent  of  intensity.  In  other 
words,  a  dose  of  500  roentgens  (a  roentgen  or  r  unit  produces  two  ionizations 
per  cubic  micron  of  tissue)  will  cause  the  same  number  of  mutations  whether 
received  over  a  period  of  20  minutes  or  20  months,  and  the  effect  is  cumulative. 
Chromosome  breaks  are  presumed  to  be  proportional  to  dose  also.  However, 
two-hit  chromosomal  aberrations  (for  example,  translocations,  whose  formation 
depends  on  the  simultaneous  occurrence  of  two  open  breaks)  show  an  intensity 
effect,  since  at  low  intensities  one  break  usually  reunites  before  another  break 
occurs.  Ultraviolet  light,  essentially  a  nonionizing  radiation,  is  also  mutagenic 
though  relatively  less  effective  at  breaking  chromosomes  than  the  ionizing 
radiations. 

The  mutagenic  properties  of  the  mustard  gases  were  discovered  during 
World  War  II,  and  since  then  a  variety  of  chemical  substances  has  been  shown 
capable  of  raising  rates  of  mutation  and  chromosome  breakage.  As  yet,  no  pattern 
is  apparent  in  the  types  of  effective  compounds,  which  include  peroxides,  for- 
maldehyde, urethane,  triazine,  diepoxide,  caffeine,  phenol,  and  also  cancer- 
producing  compounds  such  as  dibenzanthracene  and  methyl-cholanthrene. 


210  •  THE    MECHANISM    OF   EVOLUTION 

Study  of  the  effects  of  mutagenic  agents  in  combination  with  each  other 
or  with  other  agents  has  shown  a  variety  of  modifying  effects.  Infrared  alone  is 
not  mutagenic,  but  pretreatment  with  infrared  followed  by  x-radiation  raises  the 
yield  of  aberrations  above  that  of  the  same  dose  of  x-rays  alone.  On  the  other 
hand,  exposure  of  cells  to  ionizing  radiations  under  conditions  of  anoxia  gen- 
erally reduces  the  yield  of  aberrations  as  compared  to  radiation  with  oxygen 
present.  The  mutagenic  effects  of  ultraviolet  light  can  be  counteracted  by  subse- 
quent exposure  to  visible  white  light.  Chemical  substances  such  as  reducing  com- 
pounds, British  anti-Lewisite  (BAL),  and  alcohol  have  been  shown  to  protect 
cells  against  radiation  damage.  However,  even  though  such  findings  offer  the 
hope  that  some  protective  measures  can  eventually  be  developed  against  the 
physiological  and  genetic  damage  caused  by  atomic  warfare  or  other  radiation 
hazards,  the  therapeutic  consumption  of  large  quantities  of  alcohol  in  the  event 
of  an  atomic  war  has  not  yet  been  recommended. 

Mutation  Rates 

A  most  interesting  aspect  of  the  mutation  process  was  revealed  by  the 
discovery  of  the  so-called  mutation-rate  genes,  which  affect  the  mutation  rates  of 
genes  at  other  loci.  In  corn,  for  example,  the  recessive  ax  gene  (the  Ax  locus  con- 
trolling anthocyanin  production)  is  stable  in  the  presence  of  the  recessive  dt 
allele  at  the  dotted  locus.  The  dominant  Dt,  however,  induces  instability  in  the 
ax  allele,  causing  it  to  mutate  to  Ax  at  a  high  rate,  so  high,  in  fact,  that  it  is 
called  an  "ever-sporting"  gene.  (New  mutant  types  used  to  be  called  "sports" 
before  the  term  mutation  came  into  general  use.  It  seems  a  pity,  almost,  that  the 
more  colorful  word  was  not  retained.)  Another  instance  is  the  "hi"  mutant  in 
Drosophila,  which  differs  from  Dt  in  that  it  raises  mutation  rates  at  many  loci 
rather  than  just  one,  and  also  induces  chromosome  breakage.  The  existence  of 
these  mutation-rate  genes  raises  the  intriguing  possibility  that  the  mutation  rates 
in  natural  populations  can  be  controlled  by  natural  selection  by  either  favoring  or 
eliminating  these  genes. 

Mutation  is  essentially  a  random  process  in  that  it  is  not  possible  to 
predict  when  a  given  gene  will  mutate,  nor  do  mutations  occur  as  an  adaptive 
response  to  an  environmental  stimulus.  However,  it  is  not  completely  random, 
for  the  mutations  occur  within  the  framework  of  the  existing  genotype.  Further- 
more, the  same  mutation  tends  to  recur,  time  and  again,  but  different  rates  of 
mutation  prevail  at  different  loci  and  for  different  mutational  changes  at  the 
same  locus.  Hence,  all  types  of  mutations  do  not  have  the  same  probability  of 
occurrence  and  some  genes  are  more  stable  than  others,  but  all  of  them,  except 
the  ever-sporting  variety,  are  exceedingly  stable.  In  man,  for  instance,  the  muta- 
tion rate  to  the  dominant  gene  causing  aniridia,  absence  of  the  iris,  has  been  esti- 
mated at  10  per  million  gametes  or  1/100,000.  One  way  to  consider  this  fact  is 


MUTATION  •  211 

that  a  single  normal  allele  would  be  expected  to  go  through  100,000  generations, 
on  the  average,  before  it  mutated.  Another  way,  however,  equally  valid,  is  to 
state  that  a  single  ejaculate  containing  100,000,000  spermatozoa  would  be  ex- 
pected to  contain  approximately  1000  sperm  cells  carrying  new  aniridia  mutants. 
The  mutation  rate  from  the  normal  condition  to  the  sex-linked  recessive  gene 
causing  hemophilia  has  been  estimated  at  one  in  31,000  gametes;  that  to  the 
autosomal  dominant  causing  achondroplastic  dwarfism  is  approximately  one  in 
24,000. 

In  corn,  more  precise  studies  than  in  the  human  material  have  shown  a 
wide  range  of  spontaneous  mutation  rates,  as  given  below : 


gametes 

number  of 

average  per 
million 

trait 

mutation 

tested 

mutations 

gametes 

colored— >noncolored  aleurone  and  plant 

R-*r 

554,786 

273 

492 

inhibitor—*  noninhibitor  of  aleurone  color 

I-*i 

265,391 

28 

106 

purple—* red  aleurone 

Pr-^pr 

647,102 

7 

11 

starchy  — >  sugary  endosperm 

Su—>  su 

1,678,736 

4 

2.4 

yellow  — *  white  starch  in  endosperm 

Y-+y 

1,745,280 

4 

2.2 

full  — >  shrunken  endosperm 

Sh-+sh 

2,469,285 

3 

1.2 

non waxy  — >  waxy  endosperm  Wx—+wx  1,503,744 

These  figures  may  be  compared  with  those  given  above  for  man : 

average  tier 


average  per 

trait 

million  gametes 

achondroplasia 

42 

hemophilia 

32 

aniridia 

10 

It  will  be  seen  that  the  rates  per  generation  are  roughly  of  the  same 
order  of  magnitude  even  though  the  generation  lengths  are  quite  different.  The 
same  is  true  of  bacteria  and  Drosophila  with  even  shorter  generation  lengths. 
The  fact  that  species  with  generation  lengths  ranging  from  about  half  an  hour 
to  thirty  years  have  comparable  average  mutation  rates  per  locus  per  generation 
of  roughly  10"5  to  10"6  seems  to  bear  out  the  earlier  suggestion  that  mutation 
rates  are  to  some  extent  under  the  control  of  natural  selection.  If,  on  an  absolute 
time  basis,  the  bacterial  mutation  rates  prevailed  in  man,  the  human  load  of 
mutations  would  be  enormous. 

Most  of  the  mutations  that  occur  are  deleterious  and  recessive  to  the 
prevailing  types  of  genes.  These  genes,  the  "wild  type,"  are  the  favorable  muta- 
tions of  the  past,  which  have  been  preserved  by  natural  selection  and  have  in- 
creased in  frequency  until  they  have  become  the  most  frequent  type.  Thus,  any 
random  change  affecting  these  favorable  genes  has  a  much  greater  probability  of 
being  deleterious  than  it  has  of  being  more  favorable  than  the  existing  genes. 

Though  Bateson  and  Punnett  at  first  visualized  recessive  mutations  as 
complete  losses  or  deficiencies  of  the  gene  loci,  the  discovery  of  back  mutations 


212  •  THE    MECHANISM    OF    EVOLUTION 

has  made  this  idea  untenable.  Even  though  many  apparent  reverse  mutations 
have  turned  out,  on  careful  genetic  analysis,  to  be  due  to  mutations  at  entirely 
different  loci,  nevertheless,  careful  analyses  such  as  those  of  Giles  with  Neuro- 
spora  have  established  the  existence  of  true  reverse  mutations. 


Controlled  Genetic  Changes 

None  of  the  mutagenic  agents  discussed  thus  far  can  be  used  to  induce 
a  predictable  specific  mutation.  Present  techniques,  both  radiation  and  chemical, 
involve  essentially  a  shotgun  treatment,  with  the  geneticist  examining  the  pieces 
for  whatever  mutations  may  have  occurred.  This  method,  of  course,  must  be  re- 
garded as  very  crude,  and  it  would  be  highly  desirable,  especially  for  the  prac- 
tical breeder,  if  he  were  able  to  control  the  mutation  process  and  to  induce  spe- 
cific kinds  of  mutations  at  will.  At  least  one  type  of  experiment  has  given  reason 
for  hope  that  controlled  mutations  may  one  day  be  possible. 

In  the  Pneumococcus  bacteria  various  types  have  been  identified  that 
differ  in  the  type  of  polysaccharide  capsule  enclosing  the  cell.  The  encapsulated 
bacteria  form  a  smooth  colony  when  cultured.  By  mutation,  the  ability  to  form 
the  polysaccharide  capsule  may  be  lost,  and  the  unencapsulated  cells  then  form  a 
rough  colony.  Back  mutation  will  give  rise  to  encapsulated  cells,  but  the  capsule 
always  has  the  same  type  of  polysaccharide  as  the  original  type.  For  example, 

smooth           mutation              rough           mutation  smooth 

Type  I >  Type  I  >  Type  I 

However,  the  addition  of  an  extract  from  killed  bacteria  with  a  different  capsular 
type  produced  the  following  result: 

smooth           mutation  rough  extract  from  ^  smooth 

Type  I >  Type  I  Type  III  Type  III 

In  this  case,  a  predictable  change  was  induced,  but  the  active  inducing  agent  was 
not  the  Type  III  polysaccharide  itself,  but  rather  the  DNA  (desoxy ribonucleic 
acid)  from  the  Type  III  bacteria.  Bacterial  transformation,  as  this  phenomenon 
is  called,  may  not  represent  a  true  induced  mutation,  but  it  is  an  induced  directed 
hereditary  change,  and  hence  is  extremely  significant  as  a  step  toward  directed 
mutation. 

In  a  somewhat  similar  case  known  as  transduction,  genetic  material  can 
be  transferred  from  one  bacterial  strain  to  another  via  a  bacterial  virus.  The  virus 
apparently  transports  the  genes  or  a  small  chromosome  segment  from  one  bac- 
terial host  to  another  where  it  becomes  incorporated  into  the  genotype  of  the 
new  host. 


MUTATION  •   213 


Fig.  21-2.     Some  "mutants"  of  the  evening  primrose,  Oenothera  lamarckiana,  on 

which  de  Vries  based  his  mutation   theory.    Oenothera  lamarckiana  above.    The 

"mutants"  from  the  left  counterclockwise  are:  O.  gigas,  O.  albida,  O.  scintillans, 

and  O.  oblonga.  (From  de  Vries.) 


The  Mutation  Theory  of  de  Vries 

In  the  very  early  days  of  genetics  de  Vries  (1902)  proposed  the  muta- 
tion theory  of  evolution  as  an  alternative  to  the  theory  of  natural  selection,  de 
Vries  had  been  working  with  the  evening  primrose,  Oenothera  lamarckiana,  in 
which  new  and  strikingly  different  types  of  plants  occasionally  appeared,  breed- 
ing true  to  the  new  type  (see  Fig.  21-2).  On  the  basis  of  this  work,  de  Vries 


214  •  THE    MECHANISM    OF   EVOLUTION 

suggested  that  new  species  originate  as  a  result  of  these  large  discontinuous 
variations  or  mutations  rather  than  from  the  gradual  accumulation  of  numerous 
small  hereditary  differences  in  size,  shape,  color,  etc.,  by  natural  selection.  How- 
ever, his  theory  turned  out  to  be  based  on  a  variety  of  changes,  stemming  from 
the  unique  features  of  the  genome  of  Oenothera,  and  including  tetraploidy, 
trisomies,  reciprocal  translocations,  and  balanced  lethal  systems.  With  a  few  pos- 
sible exceptions,  these  hereditary  changes  did  not  represent  genie  mutations  at  all 
even  though  they  bred  true  and  remained  distinct  from  the  parental  types;  rather, 
they  were  actually  the  result  of  recombination  of  chromosomes  or  genes.  These 
spurious  mutants  in  Oenothera  are  the  result  of  a  unique  situation  not  to  be 
found  in  all  species,  and  therefore  they  cannot  serve  as  a  general  mechanism  for 
evolution. 

As  the  knowledge  of  heredity  has  increased,  mutations  of  all  degrees 
have  been  studied.  Their  effects  may  be  great,  or  they  may  be  so  small  that  re- 
fined statistical  or  genetic  methods  are  needed  to  detect  the  difference  between 
different  mutant  types.  As  an  understanding  of  the  nature  of  mutation  has  de- 
veloped, it  has  become  clear  that  de  Vries,  though  basing  his  mutation  theory  of 
evolution  on  changes  that  were  not  genie  mutations  at  all,  was  fundamentally 
correct  in  stressing  the  significance  of  mutation  to  the  evolutionary  process. 
However,  mutation  alone  cannot  account  for  evolution;  rather  it  furnishes  the 
raw  materials  on  which  other  forces  act  to  bring  about  evolutionary  change. 


SUMMARY  < 


In  a  broad  sense  mutation  implies  a  change  that  takes 
place  in  the  hereditary  material  and  does  not  arise  as  a  conse- 
quence of  recombination.  In  a  narrower  sense  mutation  is  used  to 
refer  to  a  self-duplicating  change  at  a  specific  locus.  Mutations 
form  the  raw  working  material  of  evolution,  for  the  mutation 
process  is  the  only  one  giving  rise  to  entirely  new  kinds  of 
hereditary  variation.  Because  spontaneous  mutations  are  typically 
recurrent,  it  is  possible  to  estimate  mutation  rates.  These  rates  may 
be  increased  by  various  treatments  such  as  temperature  shock, 
ionizing  radiations,  and  chemical  mutagens,  and  by  the  effects  of 
mutation-rate  genes.  Mutation  is  a  random  process  in  the  sense 
that  it  is  impossible  to  predict  when  a  given  gene  will  mutate  and 
that  mutations  do  not  occur  as  adaptive  responses  to  environ- 
mental stimuli.  However,  they  can  only  occur  within  the  frame- 
work imposed  by  the  existing  genotype.  Most  new  mutants  are 
deleterious,  presumably  because  the  prevailing  "wild  types"  are 
the  favorable  mutations  of  the  past,  preserved  by  natural  selection, 
and  any  random  change  in  these  favorable  genes  has  a  greater 
chance  of  being  harmful  than  of  having  increased  adaptive  value. 


MUTATION  •  215 

The  mutation  theory  of  evolution,  suggested  by  de  Vries  as  an 
alternative  to  natural  selection,  is  not  sufficient  alone  to  account 
for  evolution,  but  mutation  and  natural  selection  together  are 
major  factors  in  evolution. 

SUGGESTED  READING 

Demerec,  M.,  ed.,  1951.  "Genes  and  mutations,"  Cold  Spring  Harbor  Symp.  Quant. 

Biol.,  Vol.  16.  Long  Island  Biological  Ass'n,  New  York. 
Muller,   H.  J.,    1959.   "The  mutation  theory  re-examined,"  Proc.  X  International 

Congress  of  Genetics,  i;306-317. 
Stadler,  L.  J.,  1954.  "The  gene,"  Science,  120:81 1-819. 


CHAPTER 


22 

Quantitative  Inheritance 


Thus  far,  the  traits  we  have  considered  have  been  dis- 
continuous, and  the  differences  have  been  qualitative  and  could 
be  easily  determined.  A  person  is  either  red-haired  or  he  is  not 
Classifying  people  according  to  height  or  weight  is  something 
else,  for  they  are  not  just  tall  or  short,  thin  or  fat;  they  fall  into  a 
continuous  pattern  from  tall  to  short,  thin  to  fat.  In  fact,  more 
people  fall  into  the  intermediate  height  and  weight  ranges  than 
at  the  extremes.  They  must  be  measured  rather  than  classified,  and 
the  frequency  distribution  of  these  measurements  takes  the  form 
of  a  bell-shaped  normal  curve.  When,  for  example,  the  height  of 
a  group  of  college  men  was  measured,  the  frequency  distribution 
had  the  form  shown  in  Fig.  22-1. 

Such  a  population  can  be  described  in  terms  of  the  mean 
and  the  standard  deviation.  The  mean  or  average  falls  at  the 
center  of  the  normal  curve,  and  is  estimated  from  the  sample  as 

n 
where         x  =  mean 

S  =  the  sum  of 

x  =  the  measurement  on  one  individual 

n  =  the  number  of  individuals  measured 

The  standard  deviation  (s)  is  a  measure  of  the  variability  of  the 
group  and  is  computed  as 


_    IK* 


x  —  at)5 


216 


QUANTITATIVE    INHERITANCE  •  217 

More  than  99  percent  of  the  individuals  in  the  population  should  fall  within 
plus  or  minus  three  standard  deviations  from  the  mean.  The  standard  deviation 
thus  provides  a  way  of  comparing  an  individual  with  the  population  of  which 
he  is  a  part.  The  square  of  the  standard  deviation  (s2)  is  of  considerable 
theoretical  importance  in  the  study  of  variability  and  is  known  as  the  variance. 
The  standard  error  of  the  mean  (s$)  is  estimated  as 


Sx  = 


y/  n 


and  is  useful  as  an  estimate  of  the  variability  of  sample  means  in  much  the  same 
way  that  the  standard  deviation  is  an  estimate  of  the  variability  of  individuals  in 
a  sample. 


,..•••••••. 

< 

• 

•• 

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• 
• 

• 

* 

• 
• 

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• 

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> 
» 

• 



• 

• 

/ 

• 
• 
• 

• 

• 

• 
• 

•    •• 

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••     • 

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• 

• 

• 

• 

• 

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• — 

25 


20 


c 

0) 
^   10 


5* 


Ht  in  inches  58  59  60 
n        1   0   0 


62  63  64  65  66  67  68  69  70 
5   7   7   22  25  26  27  17   11 


72  73  74  75     76 
4   4   1 


Fig.  22-1.     The  normal  curve.    Height  in  man.    (Data  from  Blakeslee.) 

For  the  data  from  Fig.  22-1 

x  =  67-31  inches 

s  =  3.09  s2  =  9.56 

ss  =  0.23 

Thus,  more  than  99  percent  of  the  individuals  in  the  sample  would  be  expected 
to  have  heights  lying  within  the  limits  67.31  ±5s  or  from  58.04  to  76.58  inches, 
and  actually  only  1  in  175  lies  just  outside  this  range.  Similarly,  more  than 
99  percent  of  the  means  of  comparable  samples  would  be  expected  to  fall  within 
the  limits  67.31  ±3ss  or  from  66.62  to  68.00  inches. 

From  the  normal  curve,  it  can  be  seen  that  small  deviations  from  the 
mean  are  more  frequent  than  large,  that  negative  deviations  are  as  frequent  as 


218  •  THE    MECHANISM    OF    EVOLUTION 

positive,  and  that  very  large  deviations  are  not  due  to  chance  alone.  An  exces- 
sively fat  boy,  then,  may  be  suffering  from  thyroid  trouble,  or  simply,  like  Mr. 
Pickwick's  Joe,  from  overeating.  Thus,  quantitative  traits  are  subject  to  environ- 
mental modification,  much  more  so  than  qualitative  traits  such  as  red  hair. 

Genetics  of  Quantitative  Traits 

The  genetic  analysis  of  quantitative  traits  is  difficult  because  of  their 
continuous  nature  and  the  effects  of  the  environment,  and  for  some  time  it  was 
felt  that  a  Mendelian  explanation  was  inadequate  to  account  for  the  results  from 
crosses  involving  such  traits.  In  a  classical  cross  by  East,  for  example,  between 
Black  Mexican  sweet  corn  and  Tom  Thumb  popcorn,  the  ¥1  mean  was  interme- 
diate between  the  means  of  the  parents.  The  F2  mean  was  similar  to  the  Fx 
mean,  but  the  F2  was  considerably  more  variable  than  either  the  Fx  or  the 
parents,  the  more  extreme  F2  individuals  overlapping  the  parents  (Fig.  22-2). 

East  and  Nilsson-Ehle  independently  arrived  at  a  Mendelian  explana- 
tion for  such  results.  The  intermediacy  of  the  Fj  had  long  been  interpreted  to 
indicate  some  type  of  blending  inheritance,  but  blending  failed  to  account  for 
the  increased  variability  of  the  F2.  The  multiple  factor  hypothesis  postulated  that 
quantitative  traits  were  due  to  the  action  of  a  number  of  different  gene  pairs, 
each  cumulative  but  of  small  effect  as  compared  to  environmental  influences. 
The  intermediate  Fx  was  due  to  a  partial  or  complete  lack  of  dominance.  The 
increased  variability  of  the  F2  was  due  to  the  segregation  and  recombination  of 
the  many  gene  pairs.  For  instance,  the  above  cross  can  be  outlined  as  follows: 

Pi  AABBCCDD    X      aabbccdd 

Black  Mexican  Tom  Thumb 

1 
Fi  AaBbCcDd 

frequency 
I  (possible  distinct  genotypes 

plus  genes  and  phenotypes) 

F2           8           AABBCCDD  1 

7            AABBCCDd,  AABBCcDD,  etc.  4 

6            AABBCCdd,  AABBCcDd,  etc.  10 

5            AABBCcdd,  AABbCcDd,  etc.  16 

4            AABBccdd,  AaBbCcDd,  etc.  19 

3            AABbccdd,  AaBbCcdd,  etc.  16 

2            AAbbccdd,  AaBbccdd,  etc.  10 

1             Aabbccdd,  aaBbccdd,  etc.  4 

0             aabbccdd  1 

This  theory,  though  simplified,  has  been  very  serviceable  for  work  with  quantita- 
tive traits.  Some  of  the  more  obvious  oversimplifications  are  that  the  genes  have 


.45 
.40  H 
.35 
.30 


O-  -25^ 


|  .20 
B 

cL  .15  H 


QUANTITATIVE    INHERITANCE  •  219 
.45 


.10  H 

.05 


5         6         7         8 
Ear  length  in  cm 
TOM  THUMB  POPCORN  (60) 
.30 


13 


14        15       16       17        18        19       20       21 
Ear  length  in  cm 
BLACK  MEXICAN  SWEET  CORN  (54) 


10        11         12        13 

Ear  length  in  cm 

Ft  GENERATION  (60x54) 


16      17 


10 


16        17       18        19 


12  13  14  15 
Ear  length  in  cm 
F2  GENERATION 

Fig.  22-2.     Quantitative  inheritance  in  maize.    Ear  length  in  parents,   F1   and  F2 

generations  of  a  cross  between  Tom  Thumb  popcorn  and  Black  Mexican  sweet 

corn.    (Data  from  East  and  Hayes.) 


220  •  THE    MECHANISM    OF   EVOLUTION 

equal  and  additive  effects.  Evidence  is  available  that  multiple  factors,  also  called 
polygenes,  are  not  all  equivalent  in  their  effects  on  a  given  trait  and  that  the 
effect  of  a  given  genie  substitution  will  vary  with  different  genetic  backgrounds 
rather  than  being  simply  additive.  Hence,  contrary  to  the  multiple  factor  hypo- 
thesis, these  genes  are  neither  equal  nor  additive  in  their  effects.  The  genetic 
situation  is  obviously  complex,  and  the  environmental  influences  on  quantitative 
traits  also  make  this  type  of  trait  difficult  to  study.  However,  such  studies  are 
very  significant  both  to  the  student  of  evolution  and  to  the  practical  breeder,  for 
the  more  important  economic  traits  and  species  differences  have  both  turned  out 
to  be  of  this  type.  The  radish,  of  the  genus  Raphanus,  and  the  cabbage,  of  the 
genus  Brassica,  are  not  only  distinct  species  but  belong  to  different  genera.  When 
they  have  been  crossed,  the  leaves,  flowers,  seed  pods,  etc.,  are  intermediate  be- 
tween those  of  the  parent  species,  indicating  differences  at  many  gene  loci  of  the 
multiple  factor  type.  Sumner  obtained  similar  results  in  work  with  two  subspecies 
of  the  deer  mouse,  Peromyscus  polionotus.  The  extent  of  the  pigmented  area 
varies  considerably  between  the  subspecies  leucocephalus  as  compared  to  polio- 
notus, and  crosses  revealed  the  following  situation : 

Pi  leucocephalus  X  polionotus 

45.5  |        93.0 

Fx  68.3 

1         , 
F2  69.1 

The  F2  was  more  variable  than  the  F1}  the  typical  result  in  multiple  factor 
crosses.  These  few  examples,  to -which  the  mule  could  be  added,  illustrate  a 
principle  that  is  generally  true:  where  crosses  between  members  of  different 
taxonomic  groups  are  possible,  the  progeny  are  intermediate  for  most  traits — an 
indication  that  evolution  has  proceeded  by  the  gradual  accumulation  of  numerous 
genetic  differences. 

Multiple  factors  play  a  somewhat  different  type  of  role  when  they 
modify  the  expression  of  a  gene  of  major  effect.  In  the  familiar  black  and  white 
spotted  Holstein  dairy  cattle,  one  gene  locus  controls  spotting.  SS  and  5j-  indi- 
viduals are  self-colored;  ss  are  spotted.  However,  the  amount  of  spotting  is  influ- 
enced by  numerous  other  modifying  factors.  These  genes  are  detectable  only  in 
ss  individuals  and  have  no  other  known  effect  than  their  ability  to  modify  the 
expression  of  the  ss  genotype.  They  are  so  numerous  that  they  cannot  be  indi- 
vidually identified  or  handled  genetically,  yet  selection  by  the  breeder  can  either 
increase  or  decrease  the  amount  of  spotting. 

Heterosis 

The  American  farmer  in  recent  years  has  planted  hybrid  corn  almost 
exclusively.  This  hybrid  corn,  because  of  its  greater  sturdiness,  size,  and  yield,  is 


QUANTITATIVE    INHERITANCE  •  221 

of  greater  economic  value  than  the  varieties  grown  forty  years  ago.  Hybrids  fre- 
quently show  such  hybrid  vigor,  or  heterosis,  which  in  some  way  is  related  to 
their  increased  heterozygosity.  Hybrid  vigor  is  now  being  exploited  in  hogs, 
chickens,  and  other  species  of  plants  and  animals.  In  addition  to  its  importance 
in  breeding,  the  heterosis  phenomenon,  which  is  a  special  aspect  of  quantitative 
inheritance,  plays  a  role  in  evolution.  See  Fig.  22-3. 

Let  us  consider  a  representative  case  of  heterosis.  Corn,  which  is  usually 
cross-pollinated,  can  be  self-fertilized  to  produce  inbred  lines,  each  very  uniform, 
of  poor  quality,  and  distinct  from  the  others.  A  cross  of  two  inbreds  gives  an 
Fx  hybrid  of  greatly  increased  size  and  yield.  The  F1}  rather  than  being  inter- 
mediate between  the  inbred  parents,  has  a  considerably  greater  yield  because  of 
the  larger  plants  with  more  ears  per  stalk,  more  rows  per  ear,  and  more  kernels 
of  larger  size  per  row.  However,  this  heterosis  cannot  be  perpetuated,  for  the 
yield  in  the  F2,  F3,  and  subsequent  generations  becomes  progressively  less  with 
the  inbreeding  of  each  generation  until,  by  the  F7  or  F8,  the  vigor  is  down  to 
the  level  of  the  original  inbred  parents. 

Two  major  theories  have  been  proposed  to  explain  the  origin  of 
heterosis.  Both  are  Mendelian,  variations  of  the  multiple  factor  hypothesis;  one 
is  known  briefly  as  the  dominance  theory,  the  other  as  overdominance.  In  1917, 
D.  F.  Jones  proposed  the  theory  of  linked  favorable  dominant  genes  to  account 
for  heterosis.  He  assumed  that  the  genes  favoring  increased  vigor,  yield,  size,  etc., 
are  dominant  while  the  more  deleterious  alleles  are  recessive,  and  that  each  line 
or  variety  has  some  unfavorable  as  well  as  favorable  genes.  The  hybrid  between 
two  varieties  then  has  favorable  dominants  at  the  maximum  number  of  loci  since 
the  different  varieties  will  tend  to  carry  different  favorable  and  unfavorable 
genes. 

inbred  A      X        inbred  B 
Pi  aaBBccDDeeff      AAbbCCddEEFF 

Fx  AaBbCcDdEeFf 

However,  the  segregation  at  inbreeding  of  the  Fa  will  restore  the  homozygous 
recessive  condition  at  one  or  more  of  the  various  loci,  and  in  subsequent  genera- 
tions more  and  more  loci  will  become  homozygous  recessive  and  the  vigor  will 
accordingly  decline.  It  might  seem  possible  to  develop  a  line  carrying  only  favor- 
able dominants  in  the  homozygous  condition  with  vigor  as  great  as  that  of  the  Fx 
hybrid,  but  linkage  of  favorable  and  unfavorable  genes  on  the  same  chromosome 
makes  this  virtually  impossible.  Even  without  linkage,  if  20  or  30  gene  pairs  are 
involved  in  heterosis — probably  a  low  estimate — it  would  be  almost  impossible  to 
recover  such  a  type  from  a  population  of  manageable  size. 

It  should  be  noted  in  passing  that  inbreeding  itself  is  not  harmful. 
Cleopatra,  the  product  of  generations  of  inbreeding  among  the  Ptolemies,  is 
almost  sufficient  by  herself  to  confirm  this  statement.  The  only  effect  of  inbreed- 


222  •  THE    MECHANISM    OF    EVOLUTION 


— ^-^^— ■*— *— ^— ^l— T^^ 

2.5       4.5       6.5       8.5      10.5     12.5     14.5     16.5     18.5    20.5    22.5     24.5     26.5     28.5    30.5    32.5 

Upper  limit  of  class  in  grams 

PORTER  TOMATO 


Upper  limit  of  class  in  grams 

F,   GENERATION 


2.5      4.5       6.5       8.5      10.5     12.5     14.5    16.5     18.5     20.5    22.5    24.5     26.5    28.5    30.5    32.5 
Upper  limit  of  class  in  grams 

PON  DEROSA  TOMATO 

Fig.  22-3.     Heterosis    in    tomatoes.     Weight    per    locule    in   grams    in    Porter    and 
Ponderosa  varieties  of  tomatoes  and  in  their  Fx  hybrid.    (Data  from  Powers.) 


QUANTITATIVE    INHERITANCE  •  223 

ing  is  to  increase  homozygosity.  However,  since  it  brings  to  light  otherwise  hid- 
den deleterious  recessives,  the  effect  is  generally  harmful.  It  is  highly  possible 
that  the  superstitions,  religious  taboos,  and  legal  restrictions  about  incest  stem 
originally  from  its  frequently  dire  biological  consequences  rather  than  from  the 
more  abstruse  psychological  damage,  the  latter  due  to  fears  that  may  well  have 
developed  after  the  taboos  were  established. 

The  theory  of  interaction  of  alleles,  later  termed  overdominance,  was 
developed  by  Fisher  and  East.  The  two  theories  may  be  compared  as  shown 
below: 

dominance  AA  =  Aa  >  aa 

overdominance    A\A\  <  A\A2  >  A2A2 

In  the  latter,  the  heterozygote  is  superior  to  both  homozygotes.  Neither 
A-l  nor  A2  is  necessarily  deleterious,  but  the  heterozygote  with  two  kinds  of  alleles 
is  metabolically  superior  to  either  homozygote  with  only  a  single  allele  repre- 
sented. Under  this  theory,  heterosis  is  directly  dependent  on  heterozygosity;  -the 
greater  the  number  of  heterozygous  loci,  the  greater  the  heterosis.  With  the 
dominance  theory,  the  heterosis  is  not  directly  dependent  on  heterozygosity,  for 
it  is  possible,  theoretically  at  least,  for  the  homozygote  to  be  as  vigorous  as  the 
heterozygote.  These  two  theories  are  not  mutually  exclusive,  and  some  evidence 
has  been  adduced  in  support  of  both  of  them.  Furthermore,  it  should  be  pointed 
out  that  a  considerable  portion  of  the  observed  hybrid  vigor  may  be  attributable 
to  the  complementary  action  of  genes  at  different  loci,  brought  together  in  favor- 
able combinations  by  crossing. 

In  conclusion,  since  wild  populations  of  all  sorts  are  generally  highly 
heterozygous,  it  is  not  surprising  to  find  heterosis  as  a  normal  situation  in  many 
wild  populations.  Furthermore,  quantitative  inheritance  is  of  particular  im- 
portance in  evolutionary  studies  because  crosses  between  subspecies  and  species 
typically  reveal  polygenically  controlled  differences  between  them.  Evolutionary 
divergence  has,  therefore,  proceeded  by  means  of  the  gradual  accumulation  of 
numerous  genetic  differences. 


►  SUMMARY 


Quantitative  traits  such  as  size  or  weight  must  be  meas- 
ured rather  than  classified,  and  typically  the  frequency  distribution 
for  such  a  trait  in  a  population  takes  the  form  of  a  normal  curve. 
The  variability  is  thus  best  described  in  terms  of  the  mean  and 
the  standard  deviation,  but  does  not  lend  itself  to  simple  Mendel- 
ian  analysis.  However,  the  multiple  factor  hypothesis,  which 
postulates  a  number  of  genes,  each  of  small  effect,  has  furnished 
a  Mendelian  explanation  for  the  behavior  of  quantitative  traits  in 


224  •  THE    MECHANISM    OF    EVOLUTION 

crosses.  Hybrid  vigor,  or  heterosis,  a  special  aspect  of  quantitative 
inheritance,  is  frequently  observed  in  the  hybrid  offspring  of  rela- 
tively inbred  parents.  The  dominance  and  overdominance  theories 
of  heterosis  explain  heterosis  as  the  result  of  the  masking  of 
deleterious  recessives  or  of  the  favorable  interaction  of  alleles, 
respectively.  These  complementary  theories  give  a  genetic  expla- 
nation to  the  heterosis  phenomenon.  Quantitative  traits  and  heter- 
osis assume  particular  importance  in  the  study  of  evolution  since 
both  have  been  shown  to  play  a  significant  role  in  natural  popu- 
lations. 

SUGGESTED  READING 

Falconer,  D.  S.,  I960.  Introduction  to  quantitative  genetics.  New  York:  Ronald. 
Gowen,  J.  W.,  ed.,  1952.  Heterosis.  Ames:  Iowa  State  College  Press. 
Mather,  K.,  1949.  Biomedical  genetics.  New  York:  Dover. 


CHAPTER 


23 


Variation  in  Natural  Populations 


Some  of  the  more  fundamental  aspects  of  genetics  have 
now  been  discussed.  Our  next  problem  is  to  relate  this  informa- 
tion to  natural  populations  and  through  natural  populations  to  the 
question  of  the  origin  and  evolution  of  species.  Many  students  of 
evolution,  ecology,  paleontology,  and  taxonomy  have  long  felt 
that  the  geneticist,  cooped  up  in  his  laboratory  with  curtains 
drawn,  raising  abnormal  flies  in  bottles,  or  x-raying  them  to  pro- 
duce mutations  and  chromosomal  aberrations,  could  contribute 
very  little  to  the  understanding  of  phenomena  in  nature.  The  as- 
sortment of  freaks  that  the  geneticist  worked  with  seemed  to  have 
little  resemblance  to  the  collections  of  individuals  from  natural 
populations  that  these  other  workers  studied.  Only  recently  has 
this  viewpoint  started  to  shift,  as  closer  genetic  analysis  of  wild 
populations  has  begun  to  reveal  the  extent  of  their  genetic  vari- 
ability. Most  of  this  variability  is  concealed  in  the  form  of  hetero- 
zygous recessive  genes,  but  it  is,  nevertheless,  much  greater  in  ex- 
tent than  had  previously  been  suspected. 

You  would  hardly  need  to  be  convinced  that  the  human 
species  is  extremely  variable,  for  people  obviously  differ  in  eye 
color,  shade  of  hair,  ear  size  and  shape,  and  so  on  and  on.  How- 
ever, you  may  hesitate  before  accepting  the  statement  that  natural 
populations,  whether  of  mice,  lice,  or  rice,  tiger  lilies  or  tigers, 
are  also  quite  variable.  Yet,  wherever  adequate  genetic  analyses 
have  been  made,  natural  populations  have  been  shown  to  be 
genetically  highly  variable.  Phenotypically,  wild  populations  are 
usually  quite  uniform,  although  I  have  felt  it  necessary  to  qualify 


225 


226  •  THE    MECHANISM    OF    EVOLUTION 

this  last  statement  ever  since  I  saw,  like  an  apparition,  an  albino  "gray" 
squirrel  crossing  my  yard,  and,  while  trout  fishing  one  day  on  the  North 
Shore  of  Lake  Superior,  a  purple  millefoil  growing  in  the  midst  of  a 
patch  of  the  usual  white  type,  and  later,  white  bluebells  growing  in  the 
same  crevice  with  blue  bluebells.  These  unusual  variants,  quite  clearly,  were 
genetic,  and  a  careful  survey  of  a  wild  population  of  any  species  will  reveal  a 
number  of  individuals  phenotypically  distinguishable  from  the  usual  "wild  type." 
Since  Drosophila  is  so  well  known  genetically,  it  is  not  surprising  that  some  of 
the  best  information  of  this  type  is  derived  from  wild  Drosophila  populations. 
In  Drosophila  melanogaster,  for  instance,  two  percent  of  several  thousand  flies 
showed  visible  differences  from  the  wild  type;  these  affected  the  size,  shape,  or 
number  of  bristles,  size,  shape,  or  color  of  the  eyes,  wing  shape  or  venation,  and 
shape  of  the  legs.  On  genetic  testing,  not  all  were  due  to  mutations,  but  the 
majority  were. 


Genetic  Analysis  of  Natural  Populations 

A  more  thorough  analysis  of  the  genetic  variability  is  possible  by  ex- 
tracting a  single  chromosome  from  a  wild  population  and  making  it  homozygous 
in  order  to  reveal  its  genetic  contents  (see  Fig.  23-1).  The  general  method  used 
in  Drosophila  consists  of  crossing  a  single  wild  male  with  females  from  a  tester 
stock  (for  example,  A/B)  carrying  a  dominant  mutant  A  to  mark  one  chromo- 
some and  a  different  dominant  B  to  mark  its  homologue.  These  dominant  genes 
are  usually  lethal  when  homozygous.  The  marked  chromosomes  carry  inversions 
that  tend  to  lead  to  the  elimination  of  almost  all  crossovers.  A  single  male  show- 
ing A  and  carrying  only  one  of  the  two  chromosomes  from  his  wild  father  is 
selected  from  the  Fr  and  crossed  again  with  A/B  females.  From  among  the 
progeny  of  this  cross  the  A  males  and  A  females  are  taken  and  interbred.  In  all 
of  them  the  homologue  of  the  A  chromosome  is  identical,  descended  from  a 
single  original  wild  chromosome  without  crossing  over.  In  the  next  generation 
the  A/A  type  die  while  of  the  remaining  flies,  %  will  be  expected  to  be  A  and 
Y3  wild  type.  However,  if  the  chromosome  being  tested  carries  a  recessive  lethal, 
no  wild-type  flies  will  appear.  Reduced  viability  or  visible  effects  produced  by 
the  chromosome  are  readily  detected.  In  this  manner  genetic  analyses  of  indi- 
vidual chromosomes  from  wild  populations  have  been  conducted. 

The  analysis  of  a  series  of  second  chromosomes  from  Drosophila  in 
New  England,  Ohio,  and  Florida  showed  that  55  percent  of  these  chromosomes 
contained  lethal  or  deleterious  recessive  genes,  most  of  them  at  different  gene 
loci  and  hence  of  independent  origin.  Many  surveys  of  other  species  of  Dro- 
sophila have  produced  similar  results,  and  one  such  study  led  to  the  conclusion 
that  in  less  than  3  percent  of  the  flies  studied  was  there  no  harmful  mutation  in 
either  the  second  or  third  chromosomes.  When  the  other  three  pairs  of  chromo- 


VARIATION    IN    NATURAL    POPULATIONS  •  227 

A 


Fig.  23-1.     Generalized    method    of   genetic    analysis    of    individual    chromosomes 

from  wild  populations. 


somes  are  taken  into  account,  it  is  clear  that  there  are  practically  no  individuals 
who  do  not  carry  at  least  one  deleterious  recessive  mutant  gene.  Since  these 
deleterious  genes  are  balanced  by  their  dominant  wild-type  alleles,  a  given  pair 
of  chromosomes  may  carry  several  harmful  genes  that  are  not  expressed.  If  the 
mutants  are  closely  linked  lethals,  a  balanced  lethal  system  will  be  established, 


+ 


+  3       14 


li     +: 


+  ■ 


in  which  only  the  heterozygotes  survive.  Unless  there  is  some  means  of  detecting 
the  homozygous  lethal  zygotes,  such  a  balanced  lethal  system  will  appear  to  be  a 
true-breeding  homozygous  strain. 

Mutations  are  constantly  recurring  in  both  wild  and  laboratory  popula- 
tions, which  replenish  the  lethals  and  the  deleterious  mutants  that  are  being 


228  •  THE    MECHANISM    OF   EVOLUTION 

eliminated  from  the  population  by  natural  selection  against  the  homozygotes. 
However,  the  harmful  effects  of  these  mutants  are  apparently  not  confined  to  the 
homozygotes,  for  a  study  of  the  viability  of  individuals  heterozygous  for  lethals 
showed  them  on  the  average  to  be  4  percent  less  viable  than  the  homozygous 
wild-type  individuals.  Thus,  the  damage  wrought  by  deleterious  genes  due  to 
their  insidious  effects  on  heterozygotes  over  a  number  of  generations  may  be 
greater  than  the  single  genetic  death  of  the  homozygote. 

One  further  point  to  be  noted  and  perhaps  emphasized  is  that  the  muta- 
tions revealed  by  the  genetic  analyses  of  wild  populations  of  Drosophila  were  no 
different  in  kind  from  those  studied  by  the  geneticist  in  the  laboratory  for  many 
years.  Furthermore,  Drosophila  are  not  unique  in  carrying  large  numbers  of  con- 
cealed recessives;  they  are  observed  frequently  in  other  species  as  well.  The  most 
striking  variant  I  ever  saw  was  an  albino  snapping  turtle,  but  adequate  sampling 
of  any  species  will  reveal  some  individuals  distinctly  different  from  the  so-called 
"wild  type."  More  careful  study  will  show  that  the  extreme  types  grade  into  less 
extreme  types  and  on  into  quantitative  differences  so  that  the  variability  is  in 
degree  rather  than  in  kind. 

The  phenotypic  variation  in  wild  populations  has  frequently  been  as- 
cribed to  environmental  effects,  and  without  doubt  this  is  often  true.  A  com- 
parison of  the  growth  of  a  field  of  corn  during  a  wet  summer  and  a  dry  one  will 
reveal  how  great  an  influence  the  environment  can  have.  Hence,  there  has  been  a 
general  tendency  to  regard  all  of  the  differences  exhibited  between  populations 
of  a  species  living  in  different  habitats  to  be  nongenetic.  However,  when  repre- 
sentatives from  different  populations  are  grown  together  under  the  same  environ- 
mental conditions,  many  of  the  differences  remain.  For  an  example,  let  us  con- 
sider a  cinquefoil,  Potentilla  glandulosa,  which  grows  in  California.  As  you  go 
inland  from  the  Pacific,  this  plant  is  found  in  a  variety  of  habitats:  the  Coast 
Range,  with  low  elevation  and  a  mild  climate;  the  foothills  of  the  Sierra  Nevada 
with  both  dry  slopes  and  open  meadows  and  a  continental  climate  of  hot  sum- 
mers, cold  snowy  winters,  and  rainy  springs;  subalpine  and  alpine  habitats  up  in 
the  Sierras  with  a  short  growing  season,  cold  winters,  and  abundant  precipita- 
tion. Reciprocal  transplants  of  individuals  from  each  of  these  habitats  to  all  of 
the  others  showed  that  the  differences  between  them  were  hereditary.  The  popu- 
lations had  become  genetically  adapted  to  their  own  particular  habitats,  and 
hence,  even  though  not  far  removed  geographically,  they  belonged  to  different 
races  or  ecotypes.  Furthermore,  even  though  all  were  members  of  the  same 
species,  none  of  the  lowland  races  could  even  survive  in  the  alpine  environment. 
See  Fig.  23-2. 

When  the  different  races  were  crossed,  no  two  individuals  among  some 
1600  F2  progeny  were  alike,  and  the  minimum  number  of  genes  differentiating 
these  races  was  estimated  to  be  from  60  to  100.  Such  a  burst  of  recombination 
indicates  clearly  that  the  genes  in  one  race  differ  from  their  alleles  in  other  races, 


VARIATION    IN    NATURAL    POPULATIONS  •  229 

and  that  the  observed  differences  are  not  due  merely  to  the  direct  effects  of  the 
different  environmental  forces  operating  on  similar  genotypes. 

Chromosomal  Variation 

In  addition  to  the  genie  variability  existing  within  populations  and  be- 
tween populations  of  the  same  species,  chromosomal  rearrangements  are  found 
frequently  and  in  some  cases  regularly  in  wild  populations.  The  most  detailed 
study  of  inversions  in  nature  has  been  made  in  the  genus  Drosophila  with  the 
inversions  in  the  third  chromosome  of  D.  pseudoobscura.  Many  inversions, 
which  rearrange  the  banded  structure  in  the  salivary  chromosomes,  have  been 
identified.  The  different  arrangements  can  be  related  to  each  other  by  the  fact 
that  one  pattern  can  give  rise  to  another  by  a  single  inversion.  For  example, 

Pattern  1        ABCDEFGHIJ 

T  T 

break  break 

Pattern  2        ABCGFEDHIJ 

T        T 

break    break 
Pattern  3        ABCGEFDHIJ 

These  three  patterns  are  clearly  related  to  each  other,  though  3  and  1  are  not 
directly  related  but  only  through  2.  Three  sequences  for  their  origin  are  possible: 

1    >    2    >    3 

1    < 2    < 3 

1    < 2    >    3 

In  this  manner,  a  phylogeny  of  these  inversion  types  has  been  con- 
structed, including  more  than  20  different  inversions  and  two  other  species  as 
well,  D.  miranda  and  D.  persimilis. 

Translocations  are  also  found  in  natural  populations,  the  best-studied 
case  being  the  Jimson  weed  {Datura  stramonium) .  Datura  has  12  pairs  of  chro- 
mosomes, but  crosses  of  different  races  give  rings  of  4  or  6  chromosomes  rather 
than  12  bivalents.  The  cause  of  these  rings,  as  we  have  seen,  is  the  synapsis  of 
chromosomes  with  translocations.  At  least  7  translocations  have  been  identified 
from  different  races  of  Jimson  weed,  and  translocations  have  been  observed  in 
many  other  species  of  plants  and  animals.  The  evening  primrose  is  the  most 
spectacular  case,  having  translocations  as  a  regular  part  of  the  genetic  mechanism 
of  individuals  in  the  same  population. 

Natural  polyploids  are  especially  common  among  plants,  for  most 
genera  of  plants  have  polyploid  members.  In  the  genus  Solatium  (nightshade, 
potato,   eggplant,   and   so  on)    the  following  numbers  have  been   identified: 


230  •  THE    MECHANISM    OF    EVOLUTION 


%  «fev 

:; 


F##.  23-2.  Representatives  of  four 
subspecies  or  ecological  races  of  the 
cinquefoil,  Potent  ilia  glandulosa, 
grown  in  a  uniform  garden  at  Stan- 
ford. The  different  races  come  from 
central  California  along  a  200  mile 
transect  from  the  coast  inland  into 
the  Sierra  Nevada.  Races  shown 
from  west  to  east  are:  bottom  row, 
typica;  second  row,  reflexa;  third 
row,  hanseni;  top  row,  nevadensis. 
All  to  the  same  scale.  (Courtesy  of 
Clausen  and  Heisey.) 

In  —  24,  36,  48,  60,  72,  96,  and  120.  Polyploids  are  also  known  in  such  diverse 
groups  as  strawberries,  grasses,  lilies,  spiderworts,  cotton,  tobacco,  iris,  mints, 
willows,  and  sunflowers.  In  these  cases,  the  polyploids  are  higher  multiples  of 
some  basic  haploid  number.  In  some  cases,  the  postulated  ancestry  of  an  apparent 
allopolyploid  has  been  confirmed  by  the  experimental  resynthesis  of  the  poly- 


VARIATION    IN    NATURAL    POPULATIONS  •  231 


*€%., 


y|||| 


\  M  r  HI 


\SvV>. 


V   .-..*.      *<M-Y*«5 


*r-..'. 


^  g* 


yRmBBm 


M% 


^fX^^?'  -£ 


. /* 


ploid  from  the  diploid  ancestors.  One  such  case  is  the  synthesis  of  the  allo- 
polyploid Galeopsis  tetrahit  from  the  diploids,  G.  pubescens  and  G.  speciosa 
(Fig.  23-3). 

This  brief  survey  should  make  it  clear  that  the  genie  and  chromosomal 
changes  found  during  observation  and  experiment  in  the  laboratory  and  in  ex- 


232  •  THE    MECHANISM    OF    EVOLUTION 


Fig.  23-3.  The  first  successful  resynthesis  of  a  naturally  occurring  species, 
Galeopsis  tetrahit.  Shown  are  the  ancestral  diploid  species,  G.  speciosa 
(left)  and  G.  pubescens  (right),  and  the  artificial  tetraploid  (center)  derived 
from  them,  which  is  indistinguishable  from  wild  G.  tetrahit.    (Courtesy  of 

Miintzing.) 


perimental  plots  have  their  counterparts  in  wild  populations.  There  is  no  intrinsic 
difference  between  the  variations  seen  in  the  laboratory  and  in  the  field.  Their 
nature  and  their  causes  are  the  same,  and  the  study  of  evolution  can  safely  be 
based  on  the  knowledge  about  heredity  and  variation  gained  by  experimentation. 


SUMMARY  <- 


By  means  of  special  techniques,  natural  populations  that 
usually  appear  quite  uniform  can  be  shown  to  carry  a  sizable  store 
of  genetic  variability  in  the  heterozygous  condition.  Although 
much  of  the  variation  between  individuals  and  between  popula- 
tions may  be  environmental,  the  evidence  is  clear  that  in  most 
cases  there  is  a  genetic  component  as  well,  especially  when  the 
populations  are  living  under  different  ecological  conditions.  In 
addition  to  genie  differences,  chromosomal  variation  frequently 
forms  a  characteristic  part  of  the  hereditary  variability  of  a  species. 
Thus,  for  example,  in  many  species  translocation  or  inversion 
heterozygotes  are  routinely  found,  and  many  plants  are  clearly 
polyploid  in  origin. 


VARIATION    IN    NATURAL    POPULATIONS  •  233 

SUGGESTED  READING 

Darwin,  C,  1872.  The  origin  of  species.  New  York:  Mentor  Books  (1958). 

Dobzhansky,  Th.,  1951.  Genetics  and  the  origin  of  species,  3d  ed.  New  York: 
Columbia  University  Press. 

Mayr,  E.,  1942.  Systematic s  and  the  origin  of  species.  New  York:  Columbia  Univer- 
sity Press. 

Stebbins,  G.  L.,  1950.  Variation  and  evolution  in  plants.  New  York:  Columbia 
University  Press. 


cL&* 


CHAPTER 24  rtUtidf-tffy   'f^^fo 

chapter  ^  ^  c^^j^fJ~  '^ryrt   t^- 
Genetics  of  Populations 


Evolution  has  been  termed  "descent  with  modification," 
by  Darwin.  Further  consideration  is  needed  to  clarify  this  concept. 
The  first  question  to  answer  is  "What  is  it  that  evolves?"  It  is  not 
the  individual,  for  the  individual  lives  and  dies  with  a  fixed  geno- 
type that  does  not  change;  rather,  the  species  is  the  evolving  unit. 
Even  without  a  formal  definition  of  a  species,  it  is  nevertheless 
clear  that  a  species  consists  of  a  number  of  individuals;  it  is  a 
population,  and  evolution  is  a  population  phenomenon.  For  evo- 
lutionary change  to  occur,  a  population  with  one  set  of  hereditary 
characteristics  must  in  some  way  give  rise  to  a  population  with  a 
different  set  of  hereditary  characteristics.  Since  inherited  traits  are 
controlled  by  the  genes,  evolution  can  be  redefined  as  a  change  in 
the  kinds  or  frequencies  of  genes  in  populations.  The  problem 
then  becomes  to  discover  how  the  frequency  of  a  gene  already 
existing  in  the  population  may  change,  or  how  new  types  of 
genes,  originating  by  mutation,  become  incorporated  into  the 
population.  In  order  to  study  the  genetics  of  a  population,  it  is 
necessary  to  consider  it,  not  as  a  group  of  individuals,  but  rather 
as  a  pool  of  genes  from  which  individuals  draw  their  genotypes 
and  to  which  they  in  turn  contribute  their  genes  to  form  the  pool 
for  the  next  generation. 

Up  to  this  point  we  have  been  concerned  with  gene 
effects  in  individuals  and  with  the  results  of  controlled  matings 
between  individuals  of  specified  genotypes.  Most  knowledge  and 
prediction  in  genetics  is  based  on  this  type  of  experimentation. 
The  problem  now,  however,  is  to  consider  the  operation  of 
heredity  in  a  natural  variable  population  of  freely  interbreeding 

234 


GENETICS   OF    POPULATIONS  -235 

individuals.  How  are  the  genes  present  in  the  members  of  such  a  Mendel- 
ian  population  transmitted  and  distributed  to  succeeding  generations?  In 
order  to  understand  an  extremely  complex  situation,  it  is  best  to  study  it  in  its 
simplest  possible  terms.  By  restricting  our  attention  to  the  bare  essentials  of  events 
at  a  single  gene  locus,  we  can  discover  the  underlying  principles.  Once  estab- 
lished, there  is  no  reason  to  suppose  that  these  basic  principles  do  not  hold  in 
the  more  complex  as  well  as  in  the  simple  cases. 


The  Hardy-Weinberg  Equilibrium 

Let  us  consider  first  what  happens  in  a  population  in  which  selection, 
mutation,  and  other  evolutionary  forces  are  not  operating.  In  man,  the  ability 
to  taste  phenylthiocarbamide  (PTC)  is  inherited  as  a  simple  dominant.  Tasters 
of  PTC  are,  then,  of  two  genotypes,  TT  or  Tt;  nontasters  are  homozygous  reces- 
sive, tt.  Since  very  few  persons  are  aware  of  either  their  genotype  or  phenotype, 
marriages  occur  at  random  with  respect  to  this  trait.  People  do  not  ask  their 
potential  mates  whether  they  like  PTC,  for  they  simply  do  not  care.  There  is, 
therefore,  neither  preference  nor  avoidance  of  a  mate  because  of  his  PTC  sensi- 
tivities, and  mating  on  this  score  is  said  to  be  at  random. 

There  is  no  simple  answer  to  the  question  of  how  frequent  tasters  and 
nontasters  should  be  in  a  human  population.  There  will  be  no  classical  3:1 
Mendelian  ratio,  nor  will  the  dominant  tasters  necessarily  be  more  frequent  than 
the  recessive  nontasters,  for  there  is  no  known  selective  advantage  of  one  type 
over  the  other.  A  population  may  contain  any  proportion  of  tasters  and  non- 
tasters, depending  on  the  frequencies  of  the  dominant  and  recessive  genes.  In  a 
population  of  100  people  there  will  be,  since  they  are  diploid,  200  genes  at  the 
taster  locus.  Let  us  suppose  that  there  are  20  TT,  40  Tt,  and  40  tt.  The  frequency 

,  .  40+40  40+80 

of  gene  J  is  p  =   — — —   ==  .4.  The  frequency  of  gene  t  is  q  =  — tt:  -  =  -6. 


200 


20+60 


=  .4  and  a  — 


200 
60+60 


If  there  are  10  TT,  60  Tt,  and  50  tt,  p  =  , 

'  r  200  l  200 

Hence,  even  though  the  distribution  of  these  genes  in  individual  genotypes  is 
different,  their  frequencies  in  the  two  populations  are  identical.  If  mating  is  at 
random  in  the  former  population,  the  different  types  of  matings  will  occur  in 
proportion  to  the  frequency  of  the  various  genotypes  as  shown  below. 


9\ 

TT 

.2 

Tt 

A 

tt 
.4 

7T  .2 

TT  =  .04 

TT  =  .04 

Tt  =  .04 

Tt  =  .08 

Tt  .4 

TT=  .04 
Tt  =  .04 

TT=  .04 

Tt  =  .08 
tt  =  .04 

Tt  =  .08 

tt  =  .08 

tt  .4 

Tt  =  .08 

Tt  =  .08 
tt  =  .08 

tt=  .16 

Summing  up,  we  find  TT  = 
Tt  = 


and  still  p 
and  q 


tt  = 
24  = 
24  = 


.16 

.48 

.36 

.4 

.6 


236  •  THE    MECHANISM    OF   EVOLUTION 


But  this  method  is  too  cumbersome.  If  mating  is  truly  random,  then  the  combi- 
nation of  gametes  is  at  random,  and  it  is  possible  to  deal  directly  with  gene  fre- 
quencies in  the  gametes  to  obtain  the  same  result. 


9\ 

p=im 

.4 

f-XO 

.6 

P  -4 

^=.16 

pq=  .24 

t  .6 

pq  =  .24 

q2  =  .36 

f  +   lpq+     q2=l 
.16+  .48  +  .36=1 

(TT)      (7>)      O) 


Furthermore,  it  should  now  be  clear  that  even  the  checkerboard  is  unnecessary, 
for  the  relation  between  gene  frequency  and  genotype  frequency  can  be  expressed 
as  the  binominal  (p  +  q)2  =  1.  From  the  binominal  expansion,  it  is  clear  that 
in_.a  large  random  mating  population  not  only  the  gene  frequencies  but  also  the 
genotype  frequencies  will  remain  constant.  In  a  random  mating  population  with 
p  —  A  and  q  =  .6,  the  equilibrium  frequencies  will  be  TT  ~  16  percent,  Tt  = 
48  percent,  and  tt  =  36  percent. 

It  should  be  noted  that  if  only  the  frequency  of  the  homozygous  reces- 
sive class  is  known,  the  frequency  of  the  recessive  gene  can  be  calculated.  For 
example,  if  9  percent  of  a  human  population  has  red  hair,  then 

f 


far)     =  ?2 

=  0.09 

Also,  if  D  =  /(RR) 

fa)      =1 

=  V^09  =  0.3 

H  =  /(Rr) 

fOO    =p 

=  1  -  n  =  0.7 

R  =  far) 

fQRK)  =  p* 

/(Rr)   =  in 

=  0.49 

-  2(.3)  (.7)  =  0.42 

Then     p   =  D  +  H+R 

j                   R+  W 
and       ?=D  +  H+R 

Thus  can  the  entire  population  be  described.  Perhaps  the  most  surprising  fact  to 
emerge  is  that  42  percent  of  a  random  mating  population  must  be  heterozygous 
carriers  of  the  recessive  gene  that  is  expressed  homozygously  in  only  9  percent  of 
the  population.  This  disparity  becomes  even  greater  for  the  less  frequent  reces- 
sives.  For  instance,  if  q2  =  0.01,  2pq  =  0.18;  if  q2  =  0.0001,  2pq  =  0.0198. 
This  equilibrium  is  known  as  the  Hardy-Weinberg  equilibrium,  after 
the  men  who  independently  derived  the  equation  and  understood  its  implica- 
tions. To  state  the  law  more  explicitly,  in  a  large,  randomly  mating  population, 
in  the  absence  of  mutation  and  selection,-  the  relative  frequencies  of  the  genes 
will  tend  to  remain  constant  from  generation  to  generation.  Darwin,  because  of 
his  belief  in  blending  inheritance,  thought  that  variability  decreased  each  genera- 
tion and  had  to  be  constantly  replenished.  However,  from  the  Hardy-Weinberg 
equation,  it  is  clear  that  so  long  as  TT,  Tt,  and  //  survive  and  reproduce  equally, 


GENETICS   OF    POPULATIONS  •  237 

the  variability  in  the  population  will  be  unchanged,  and  the  equilibrium  then  is 
a  conservative  factor  in  evolution.  In  fact,  evolution  can  now  be  redefined  quite 
simply  as  a  shift  in  the  Hardy -Weinberg  equilibrium.  The  factors  responsible  for 
bringing  about  such  shifts  are  mutation,  natural  selection,  migration  or  gene 
flow,  and  random  genetic  drift,  each  of  which  we  shall  consider  in  greater  detail 
here  and  in  following  chapters. 


Mutation 

Let  us  first  examine  the  effects  of  mutation  on  gene  frequencies.  Sup- 
pose that  T  mutates  to  /  at  the  rate  of  1  in  10,000  gametes  per  generation.  Muta- 
tion can  then  be  said  to  be  causing  an  increase  in  the  frequency  of  /,  for  the 
proportions  of  T  and  t  are  changing.  J[n  due  time,  if  no  other  force  intervenes, 
no  T  genes  would  be  left  at  all,  and  the  entire  population  would  be  //.  Such  a 
change  would  be  very  slow  and  very  unlikely,  but  theoretically  mutation  pressure 
alone  could  bring  about  evolution,  in  this  case  eliminating  the  taster  gene. 

However,  reverse  mutations  also  can  occur,  usually  at  different  rates. 
Suppose  that  /  mutates  to  T  at  the  rate  of  5  per  100,000. 

Let  u  =  T    ->    t   =  0.00010 

v  =  t     ->     T  =  0.00005 

Then  the  change  in  frequency  of  T  (Ap)  will  equal  the  net  change  brought 
about  by  these  opposed  mutation  rates. 

increase  in  T  =  vq 
decrease  in  T  =  up 


Ap  =  vq  —  up 

Since  the  reverse  mutations  are  occurring,  the  population  can  never  be- 
come homozygous  for  one  type  of  allele.  Hence,  an  equilibrium  will  be  estab- 
lished at  the  point  where  the  number  of  mutations  from  T— >/  just  equals  the 
number  of  mutations  from  /->T;  in  other  words,  when  Ap  =  vq  —  up  —  0. 
This  equation  can  then  be  transformed  as  follows : 

vq  =  up 
v(l  -p)  =  up 

v  —  vp  =  up 
up  +  vp  =  V 
p(u  +  v)  =  V 


4i 


A    _         vt| 
P   ~   U+  V 


238  •  THE    MECHANISM    OF   EVOLUTION 


it  should  be  noted  that  the  equilibrium  value  of  p  is  dependent  only  on 
the  mutation  rates  and  is  independent  of  the  initial  gene  frequencies,  which  may 
range  anywhere  then  from  p  =  0  to  p  .==  1.  For  the  rates  given  above, 


a  =     0.00005 

V      0.00010  +  0.00005  ~  °'333 

q  =   0.667 


T^t  lhe5,wi11  -be  twice  as  many  ^cessive  /  genes  mutating  half  as  often  as  the 
dominant'  f  genes,  and  the  result  is  an  equilibrium  since  the  absolute  numbers 
of  mutations  are  equal. 

Even  though  evolutionary  change  due  to  the  action  of  mutation  pressure 
is  theoretically  possible,  the  course  of  evolution  is  not  controlled  to  any  great 
extent  by  mutation.  Mutation  is  a  limiting  factor  rather  than  a  controlling  factor 
in  evolution. 


SUMMARY  < 


The  frequency  of  a  gene  may  be  denned  as  the  propor- 
tion that  a  given  allele  forms  of  the  total  of  all  the  different 
kinds  of  alleles  at  this  locus  in  the  population.  Random  mating 
occurs  when  any  male  in  a  population  has  an  equal  chance  of 
mating  with  any  female.  Hardy  and,  Weinberg  showed  that  in  a 
large,  randomly  mating  population,  in  the  absence  of  mutation 
and  selection,  the  gene  frequencies  will  remain  constant,  and  the 
-I5"itic  variabiiity  thus  is  conserved.  However,  if  mutations  occur, 
mutation  pressure  will  tend  to  cause  shifts  in  gene  frequency! 
Where  reverse  mutations  also  occur,  a  new  equilibrium  will  be 
established  that  is  solely  determined  by  the  mutation  rates. 

SUGGESTED  READING 

Cold  Spring  Harbor  Symp.  Quant.  Biol.,  Vol.  20,  1955.  "Population  genetics."  Long 
Island  Biological  Assn.,  New  York. 

Haldane,  J.  B.  S.,  1932.  The. causes  of  evolution.  New  York:  Harper. 

Lerner,  I.  M.,  1950.  Population  genetics  and  animal  improvement.  New  York:  Cam- 
bridge University  Press. 

Li,  C.  C,  1955.  Population  genetics.  Chicago:  University  of  Chicago  Press. 


CHAPTER 


25 


Natural  Selection 


The  primary  factor  controlling  the  course  of  evolution  is 
natural  selection.  We  have  already  discussed  the  Darwinian  con- 
cept of  natural  selection,  which  assumed  a  population  more  or  less 
stable  numerically  with  a  reproductive  rate  far  higher  than  neces- 
sary to  ensure  the  maintenance  of  the  population's  size.  Because 
the  population  is  variable,  the  ensuing  deaths  occur  more  fre- 
quently among  the  less  well-adapted  individuals,  and  the  better 
adapted  types  survive.  Darwin  placed  emphasis  on  predation  and 
on  competition,  and  to  many,  natural  selection  came  to  signify  a 
concept  of  nature,  red  in  tooth  and  claw.  Another  aspect  of 
Darwinism,  neglected  in  recent  years,  was  his  concept  of  sexual 
selection  due  either  to  male  competition  or  female  preference. 

The  modern  concept  of  natural  selection  involves  a  subtle 
change  in  emphasis  from  differential  survival  to  differential  repro- 
duction. From  the  standpoint  of  evolution,  it  matters  little 
whether  an  individual  survives  to  the  age  of  2  or  to  102;  if  he 
dies  without  offspring,  his  genes  are  lost  from  the  population. 
Any  and  all  factors  that  bring  about  differential  reproduction — 
the  production  of  more  progeny  by  one  hereditary  type  in  propor- 
tion to  its  numbers  than  by  the  other  types — are  factors  in 
natural  selection.  Included  among  these  factors  are  survival  and 
longevity,  fertility  and  fecundity,  competition  and  cooperation, 
disease  and  parasite  resistance,  food  requirements,  physiological 
tolerances,  sexual  selection,  color  patterns,  behavior  patterns,  and 
so  on  and  on.  To  the  extent  that  any  of  these  factors,  trivial  or 
major,  affects  reproductive  fitness,  they  have  adaptive  value;  and 


239 


240  •  THE    MECHANISM    OF   EVOLUTION 

to  the  extent  that  the  differences  are  controlled  by  genes,  the  favorable 
genes  will  increase  in  frequency  while  the  less  favorable  genes  will  decline 
in  frequency  each  generation.  The  net  effect  is  the  production  of  organ- 
isms well  adapted  to  survive  in  their  particular  environments.  Since  many, 
many  selective  pressures  operate,  it  is  clear  that  the  organism  must  make 
some  adjustment  to  all  of  them.  Hence,  the  final  phenotypes  are  compromises 
that  permit  the  organism  to  make  the  best  possible  adjustment  to  all  the  various 
selection  pressures,  but  no  one  adaptation  is  apt  to  be  perfect.  Natural  selection, 
then,  brings  about  adaptation;  it  may  be  to  a  changing  environment,  or  it  may 
be  an  improvement  in  the  existing  adaptations  to  a  fairly  stable  environment. 
Evolution  may  thus  be  thought  of  also  as  successive  or  perhaps  in  some  cases 
progressive  adaptation. 

A  great  deal  has  been  written  about  the  theory  of  natural  selection.  It 
has  been  hailed  as  a  monumental  advance,  but  it  has  also  been  severely  criticized 
and  even  regarded  as  completely  erroneous.  We  cannot  hope  to  pursue  all  of  the 
avenues  open  to  discussion,  but  we  can  point  out  that  the  basis  of  many  of  the 
objections  seems  to  be  the  difficulty  in  visualizing  how  such  enormously  complex 
systems  as  the  human  eye,  the  electric  organ  in  fishes,  the  insect  societies,  and 
the  adaptively  appropriate  patterns  of  instinctive  behavior  could  have  arisen  as 
the  result  of  gradual  changes  emanating  from  such  an  apparently  simple  process 
as  differential  reproduction.  The  fault,  however,  lies  more  with  the  imagination 
than  with  the  process  of  natural  selection,  for  selection  almost  inevitably  tends 
toward  the  improvement  of  adaptation,  and  these  examples  represent  some  sort 
of  adaptive  pinnacle.  Although  a  detailed  history  of  the  origin  of  many  of  the 
more  bizarre  adaptations  is  not  yet  possible,  it  is  by  no  means  impossible  that  this 
history  may  eventually  be  learned. 

That  natural  selection  gave  rise  to  a  brutal  concept  of  nature  made  the 
theory  of  natural  selection  distasteful  or  even  unacceptable  to  many  people.  The 
idea  of  competition  or  the  struggle  for  existence  was  regarded  as  a  threat  to  any 
higher  concept  of  man  or  of  nature.  Distasteful  or  not,  predation,  competition, 
and  parasitism  are  biological  facts  of  life.  Anyone  who  has  spent  any  time  in  the 
field  realizes  that  death  is  a  very  casual,  commonplace  affair  among  living  things. 
Predators  live  at  the  expense  of  their  prey;  parasites,  though  less  demanding,  at 
the  expense  of  their  hosts.  Members  of  the  same  species  may  compete  for  food, 
space,  light,  or  other  essentials.  In  fact,  intraspecific  competition  may  be  even 
more  severe  than  the  competition  between  different  species.  In  a  crowded  group 
of  seedlings  only  a  few  will  survive  the  competition  for  light  and  space.  This 
contest  is  bloodless  but  fatal  nonetheless  to  the  losers.  Similarly,  under  crowded 
conditions  the  growth  of  small  tadpoles  is  inhibited  by  the  presence  of  larger 
tadpoles  of  the  same  species,  and  they  eventually  die  despite  the  presence  of 
abundant  food.  We  may  be  repelled  by  the  garter  snake  that  engulfs  a  living 
leopard  frog  inch  by  inch,  or  by  the  leech  that  drains  its  blood,  leaving  it  in  a 


NATURAL    SELECTION  •  24 1 

moribund  condition,  but  this  is  their  normal  way  of  life.  Thus  natural  selection 
does  involve  a  struggle  for  existence,  and  attempts  to  gloss  over  this  fact  do  an 
injustice  to  the  concept. 

On  the  other  hand,  to  regard  selection  as  nothing  more  than  a  bitter 
struggle  to  survive  is  just  as  erroneous,  for  biological  success  depends  on  many 
factors  in  addition  to  escaping  death.  Cooperative  behavior  may  also  contribute 
to  reproductive  fitness,  and  may  increase  as  the  result  of  natural  selection.  Care 
of  the  young  in  birds  and  mammals,  division  of  labor  in  colonial  species  such  as 
protozoans,  coelenterates,  and  insects,  and  the  complex  group  behavior  of  fishes, 
birds,  and  mammals  have  all  arisen  during  the  course  of  evolution.  In  most  cases 
they  clearly  are  adaptive  and  contribute  directly  or  indirectly  to  reproductive 
fitness,  and  therefore  must  have  been  favored  by  and  developed  under  the  influ- 
ence of  natural  selection.  Thus,  natural  selection  must  be  regarded  as  being 
responsible  not  only  for  the  unending  struggle  for  existence  but  also  for  many 
of  the  forms  of  altruistic  behavior.  In  some  of  these  cases,  the  behavior  has  dire 
consequences  for  the  individual — for  example,  the  bee,  which  dies  once  it  has 
stung  an  invader — but  if  the  chances  of  survival  of  the  colony  are  thereby  im- 
proved, this  behavior  will  be  favored  by  selection. 

Natural  selection  in  itself  does  not  admit  of  being  judged  as  good  or 
evil.  We  may  regard  its  consequences  as  either  good  or  bad,  but  they  flow  from 
the  sole  criterion  in  selection,  reproductive  fitness.  Those  factors,  whatever  their 
nature,  that  increase  fitness  will  tend  to  be  favored  by  natural  selection;  those 
decreasing  it  will  tend  to  be  eliminated. 

Artificial  Selection 

Since  there  are  sometimes  questions  or  doubts  as  to  the  efficacy  of  selec- 
tion, it  may  be  well  to  consider  some  examples  of  the  operation  of  selection.  A 
magnified,  if  somewhat  distorted,  view  of  evolution  is  obtained  from  an  exami- 
nation of  the  results  obtained  by  artificial  selection.  The  changes  wrought  by  man 
in  developing  new  breeds  are,  strictly  speaking,  evolutionary  changes,  since  a 
population  with  a  new  set  of  hereditary  traits  is  derived  from  an  ancestral  popu- 
lation; but  they  are  on  a  small  scale  and  are  directed  toward  man's  benefit  or 
amusement  rather  than  that  of  the  species.  Certainly  no  dachshund  or  Pekingese 
would  be  likely  to  consider  himself  especially  well  equipped  to  make  a  go  of  it 
on  his  own.  A  well-documented  history  of  the  development  of  a  new  breed  of 
animals  is  that  of  the  Santa  Gertrudis  cattle  on  the  fabulous  King  Ranch  in 
Texas.  The  ranch  is  in  southern  Texas  where  ordinary  beef  cattle — such  breeds 
as  Shorthorn,  Aberdeen  Angus,  and  Hereford — did  not  thrive  in  the  semi- 
tropical  rather  arid  climate,  for  they  were  bothered  by  the  heat  and  ticks  and  did 
not  grow  well  on  the  available  grasses.  The  Brahma  cattle  of  India  thrived  in 
this  climate,  but  were  of  poor  quality.  Crosses  and  back-crosses  of  Shorthorn  and 


242  •  THE    MECHANISM    OF   EVOLUTION 

Brahma,  accompanied  by  selection  for  the  desired  beef  qualities  and  ability  to 
withstand  the  climate,  ultimately  produced  a  population  with  approximately 
7/s  of  its  gene  pool  derived  from  the  Shorthorns  and  l/g  from  the  Brahmas  (see 
Fig.  25-1).  This  new  breed  is  heat  and  tick  resistant  and  gains  better  on  grass 
feeding  than  any  other  breed.  A  couple  of  footnotes  may  be  added  to  this  story. 
Dissatisfied  with  the  type  of  grass  on  their  range,  the  owners  of  the  King  Ranch 
developed  new  varieties  of  grass  and  reseeded  vast  areas  of  the  ranch  with  the 
improved  type.  Furthermore,  their  success  in  selecting  and  breeding  horses  for 
their  ability  to  run  faster  than  other  horses  has  paid  off  at  the  Kentucky  Derby 
and  elsewhere.  The  success  of  breeders  in  all  instances  is  due  basically  to  chang- 
ing the  frequencies  or  types  of  genes  and  gene  combinations  in  the  population 
of  animals  or  plants  with  which  they  are  working.  These  changes,  secured  by 
artificial  selection,  are  brought  about  by  the  differential  reproduction  of  the 
favored  types. 

Selection  for  Resistance 

The  Santa  Gertrudis  cattle  have  been  developed  within  the  past  50 
years,  and  many  other  evolutionary  changes  in  this  interval  can  be  cited.  The 
introduction  of  chemotherapeutic  agents  and  antibiotics  was  followed  by  the 
origin  of  strains  of  bacteria  that  were  resistant  to  these  agents;  for  instance, 
strains  resistant  to  the  various  sulfas,  terramycin,  aureomycin,  penicillin,  and 
streptomycin  are  known.  Moreover,  strains  of  bacteria  actually  dependent  on 
streptomycin  for  normal  growth  have  been  discovered.  These  changes  are  the 
result  of  the  drug  having  killed  all  of  the  microorganisms  except  those  carrying 
mutations  to  resistance,  which  then  become  progenitors  of  the  resistant  strains. 
The  mutations  have  been  shown  to  be  random  and  not  produced  as  a  specific 
result  of  treatment  by  the  antibiotic,  for  by  suitable  techniques,  mutations  to 
resistance  have  been  isolated  in  bacteria  never  exposed  to  the  antibiotic  at  all. 
These  facts  lead  to  caution  in  hailing  any  new  wonder  drug  as  the  final  solution 
for  any  particular  disease,  for  the  possibility  always  exists  that  the  disease  organ- 
ism will  mutate  to  resistance.  Furthermore,  the  indiscriminate  use  of  any  anti- 
biotic is  inadvisable  simply  because  it  will  increase  the  frequency  of  the  resistant 
mutants  in  the  bacterial  population  and  make  the  disease  more  difficult  to  control 
if  most  infections  are  due  to  resistant  rather  than  susceptible  organisms.  Therapy 
has  been  directed  toward  using  combinations  of  drugs,  since  the  chances  of  in- 
dependent mutations  to  resistance  to  two  or  more  antibiotics  in  a  single  bacterial 
cell  are  vanishingly  slight. 

Hydrogen  cyanide  is  commonly  thought  of  as  one  of  the  deadliest 
poisons,  yet  resistant  strains  of  the  scale  insects  attacking  citrus  fruits  have 
evolved.  Similarly,  the  widespread  use  of  DDT  caused  in  insect  populations  a 
selection  pressure  that  led  to  the  development  of  resistant  strains  of  mosquitos, 


Fig.  25-1.     The   genesis   of   a   new   breed   of   beef   cattle.     Hybridization   between 

Brahmas   (above),  Shorthorns   (center)    followed  by  selection  produced  the  Santa 

Gertrudis  breed  (below).    (Courtesy  of  Snyder  and  David.) 


244  •  THE    MECHANISM    OF   EVOLUTION 

house  flies,  and  body  lice.  They  have  appeared  in  many  different  parts  of  the 
world,  often  within  two  or  three  years  of  the  introduction  of  DDT. 

Bacteriophages  are  viruses  that  attack  and  destroy  bacteria.  Bacteria  that 
are  resistant  to  phage  can  arise  by  mutation,  but  the  virus  can  also  mutate  to 
forms  able  to  attack  the  previously  resistant  bacteria.  A  similar  situation  exists  in 
wheat-stem  rust.  As  plant  breeders  develop  new  varieties  of  wheat  that  are  re- 
sistant to  the  currently  prevalent  strains  of  rust,  new  mutant  strains  able  to  attack 
the  resistant  wheat  increase  sharply  in  frequency  until  a  new  outbreak  of  stem 
rust  occurs.  The  plant  breeder  must  try  to  keep  one  jump  ahead,  but  as  things 
stand,  he  is  not  likely  to  work  himself  out  of  a  job.  These  situations  involving 
two  different  species  are  more  complex  because  both  host  and  pathogen  (the 
disease-causing  agent)  are  capable  of  evolution,  and  each  exerts  a  selective  pres- 
sure on  the  other. 


The  Baldwin  Effect 

A  great  deal  still  remains  to  be  learned  about  the  ways  in  which  natural 
selection  operates  to  bring  about  adaptation,  for  it  is  a  subtle  as  well  as  a  power- 
ful force.  Furthermore,  the  appeal  of  Lamarckianism  has  persisted  because  it  has 
seemed  that  many  of  the  more  remarkable  adaptations  could  have  arisen  only  in 
direct  response  to  the  environment  or  to  the  needs  of  the  organism  rather  than 
by  the  operation  of  natural  selection  on  random  mutations.  Some  recent  experi- 
ments by  Waddington  on  what  is  known  as  the  Baldwin  effect  have  been  most 
revealing.  A  number  of  wild-type  fruit  flies  were  subjected  to  temperature  shock 
during  development.  As  a  result  of  this  treatment  some  of  these  flies  were  cross- 
veinless.  The  crossveinless  condition  of  the  wings  was  not  due  to  mutations  in- 
duced by  the  heat  treatment,  however,  for  untreated  progeny  of  these  flies  were 
wild  type  and  could  be  shown  not  to  carry  a  crossveinless  mutation.  Such  an 
environmentally  induced  condition  that  simulates  the  phenotype  of  a  genetic 
mutant  is  known  as  a  phenocopy.  Nevertheless,  the  crossveinless  flies  were  bred 
together,  the  offspring  given  heat  shock  during  development,  and  the  crossvein- 
less offspring  again  selected  and  interbred  over  a  period  of  several  generations. 
After  about  15  generations  of  selection,  the  heat  treatment  was  discontinued,  but 
crossveinless  flies  still  continued  to  appear  in  these  stocks. 

At  first  thought,  this  result  seems  clearly  to  indicate  Lamarckian  in- 
heritance of  acquired  characteristics.  Actually  it  does  not,  but  it  may  serve  to 
reconcile  to  some  extent  Lamarckianism  with  the  theory  of  natural  selection.  In 
the  first  place,  the  initial  wild- type  stock  had  not  been  selected  or  inbred  and  was 
therefore  undoubtedly  heterozygous.  Among  this  array  of  genotypes  were  some 
that  could  produce  the  crossveinless  phenotype,  but  only  under  the  unusual  en- 
vironmental conditions  provided  by  the  temperature  shock.  When  these  genes 
were  brought  to  expression,  selection  then  became  possible.  Experiments  with  the 


NATURAL   SELECTION  •  245 

crossveinless  stock  resulting  from  selection  showed  that  the  crossveinless  condi- 
tion was  controlled  by  polygenes  or  multiple  factors  rather  than  by  a  single  gene 
locus.  Therefore,  selection  over  a  number  of  generations  had  simply  increased 
the  frequency  of  these  genes  in  the  population  to  the  point  where  individual 
genotypes  carried  enough  of  them  to  cause  the  crossveinless  phenorype  even  in 
the  absence  of  temperature  shock.  In  other  words,  it  could  be  said  that  selection 
had  lowered  the  threshold  for  crossveinless.  It  should  be  noted  that  even  the 
ability  to  produce  the  so-called  phenocopies  was  not  independent  of  the  geno- 
type. In  these  experiments,  a  mechanism  has  been  revealed  by  which  the  re- 
sponses of  individuals  to  new  environmental  pressures  have  been  incorporated 
through  natural  selection  into  the  population  as  a  whole.  Thus  could  the  transi- 
tion from  individual  physiological  adaptation  to  population  genetic  adaptation 
be  made.  The  distinction  between  these  two  types  of  adaptation  is  obviously  not 
clear-cut,  because,  just  as  the  adaptation  of  a  population  to  its  environment  is 
determined  by  its  genetic  composition,  the  adaptive  responses  possible  to  an  indi- 
vidual are  also  controlled  by  his  genotype.  Therefore,  even  though  many  adaptive 
changes  may  appear  Lamarckian,  they  may  nevertheless  have  a  completely  reason- 
able explanation  under  the  theory  of  natural  selection. 


The  Theory  of  Selection 

With  these  examples  in  mind,  let  us  now  consider  the  way  in  which 
gene  frequencies  change  because  of  selection.  The  theory  of  selection  is  very 
simple.  Suppose  that  A  and  a  alleles  are  present  in  a  population  with  equal  fre- 
quency, but  that  only  99  a  genes  are  transmitted  to  the  next  generation  for  every 
100/1.  The  recessive  a  gene  is  therefore  at  a  slight  selective  disadvantage  to  the 
dominant.  The  selection  coefficient,  j",  is  a  measure  of  this  disadvantage  and  is 
obtained  as  follows: 

1  -  s        99 


1  100 

s  =  0.01 

Most  selection  pressures  operate  on  the  diploid  or  zygote  phase  rather 
than  on  the  haploid  or  gametic  stage.  A  common  type  of  zygotic  selection  is  that 
against  deleterious  recessive  homozygotes  with  the  homozygous  dominants  and 
the  heterozygotes  equally  viable.  For  this  situation  the  change  in  frequency  of 
the  dominant  A  gene  is  calculated  as  follows : 


genotype 

AA 

Aa 

aa 

total 

rrequency  before  selection 

f 

ipq 

42 

1 

frequency  after  selection 

f 

2pq 

q\l  -  ,3 

1  -  sq2 

246  •  THE    MECHANISM    OF   EVOLUTION 

Here,  s  measures  the  selective  disadvantage  of  the  aa  type. 

Ap  =  pi  —  p  p  =  /(/4)  in  generation  0 

p2  +  pq  pi  ==  f(A)  in  generation  1 


Pl=  1-  sf 


z  t±n _  t=   m2 

1  -  sq2       F       1  -  sq' 


If  sq2  is  small,  the  denominator  is  essentially  equal  to  1,  and  further  simplifica- 
tion is  possible  to 

Ap  =  spa2 

If  s,  p,  or  q  is  small,  selection  will  act  only  very  slowly.  Therefore,  selection  pres- 
sures are  most  effective  at  intermediate  gene  frequencies.  From  the  equation  it  is 
clear  that  selection  will  have  no  effect  at  all  if  s,  p,  or  q  equals  zero.  In  other 
words,  one  allele  must  have  a  selective  advantage  and  both  alleles  must  be 
present  in  the  population  for  selection  to  operate.  Hence,  selection  is  ineffective 
in  a  homozygous  population,  no  matter  how  great  the  environmental  variation 
may  be.  As  early  as  1910,  Johannsen  showed  experimentally  the  futility  of  selec- 
tion on  environmental  variation.  As  a  result,  Darwin's  ideas  on  selection  have 
been  modified  and  clarified,  for  he  did  not  make  a  clear  distinction  between 
hereditary  and  environmental  variation  and  believed  natural  selection  could  act 
on  both.  He  was  inclined  to  accept  Lamarckian  inheritance  of  acquired  charac- 
ters, though  at  times  he  also  seemed  to  have  some  reservations  about  the  possi- 
bility that  environmentally  induced  changes  could  become  hereditary. 

If  selection  is  directed  against  a  deleterious  dominant,  the  gene  is  ex- 
pressed and  exposed  to  selection  in  both  AA  and  Aa  individuals.  If  no  dominant 
individual  leaves  progeny,  the  gene  will  be  eliminated  except  for  new  mutations, 
in  a  generation.  Even  if  selection  is  not  complete,  it  is  still  very  effective,  for  all 
of  the  dominant  genes  are  exposed  to  selection.  It  is  for  this  reason  that  deleteri- 
ous dominant  mutations  are  so  rarely  observed  in  wild  populations,  and  a  fair 
proportion  of  those  seen  arise  from  new  mutations. 

On  the. other  hand,. selection  against  a  harmful  recessive  gene  is  consid- 
erably less  effective.  The  gene  is  carried  by  both  Aa  and  aa,  but  the  full  force  of 
selection  acts  only  on  the  aa  individuals.  Since  the  defective  homozygotes  aa  are 
normally  less  frequent  than  the  heterozygotes  Aa,  the  frequencies  being  as  q2 
{aa)  is  to  2pq  (Aa),  a  large  proportion  of  the  deleterious  recessives  are  not  ex- 
posed to  selection.  Furthermore,  the  less  frequent  a  becomes,  the  greater  the 
proportion  of  the  recessives  carried  by  the  heterozygotes,  and  hence  the  less 
effective  selection  becomes.  Even-recessive  lethals  may  be  present  in  a  fairly  high 
frequency,  for  when  no  recessive  homozygotes  survive  or  reproduce,  affected 
'  \  individuals  will  continue  to  appear  as  the  offspring  of  heterozygous  normal 
parents. 


NATURAL    SELECTION  •  247 

Selection  and  Mutation 

If  selection  against^  an  unfavorable  recessive  were  to  continue  over  a 
long  period  of  time,  eventually  the  recessive  might  be  expected  to_be  eliminated 
entirely  from  the  population.  However,  recurrent  mutation  will  periodically  add 
additional  recessives  to  the  population  before  the  recessive  is  completely  gone. 
The  forces  of  selection  pressure  and  mutation  pressure  will  therefore  tend  to  be 
opposed  under  these  circumstances,  and  an  equilibrium  between  these  opposing 
forces  will  be  established.  Since 

Ap  =  spq2  —  up 

where  spq2  is  the  effect  of  zygotic  selection  against  the  homozygous  recessive  aa 
and  u  is  the  mutation  rate  from  A  to  a,  then  at  equilibrium 

Ap  =  spq2  -  up  =  0 
spq2  =  up 

and  a2  —  - 

s 

Thus  the  frequency  of  appearance  of  the  homozygous  recessive  type  aa  (q2)  is 
determined  by  the  relationship  between  the  mutation  rate  and  the  selection  co- 
efficient. In  the  case  of  a  recessive  lethal  s  equals  1,  and  q2  =  u  directly.  For 
example,  if  one  person  in  40,000  dies  owing  to  a  homozygous  recessive  lethal 
condition,  the  mutation  rate  to  the  recessive  also  equals  1/40,000.  Moreover,  q  = 
1/200,  p  =  199/200,  and  2pq,  the  frequency  of  the  heterozygotes  (Aa),  equals 
398/40,000,  or  approximately  1  percent.  Thus  even  though  the  gene  is  lethal, 
less  than  1  percent  of  these  lethal  genes  are  exposed  to  selection  each  generation, 
and  their  frequency  in  the  population  may  remain  surprisingly  high. 

Evolutionary  change  comes  about,  then,  as  a  result  of  the  joint  effects  of 
mutation  and  natural  selection.  New  kinds  of  genes  originate  in  a  population  by 
mutation  and  may  increase  in  frequency  because  of  either  recurrent  mutation  or 
chance  events,  for  selection  is  relatively  ineffective  at  extremely  low  gene  fre- 
quencies. As  gene  frequencies  increase,  selection  becomes  increasingly  important 
in  determining  the  ultimate  fate  of  the  genes  in  the  population.  Withoutjihe 
genetic  variability  originally  supplied  by  mutation,  natural  selection  is  powerless 
Jx)j3perate.  Without  the  sifting  and  winnowing  of  natural  selection,  mutation 
pressures  would  soon  reduce  a  population  to  an  array  of  freaks. 


248  •  THE    MECHANISM    OF   EVOLUTION 

SUMMARY  < 


The  essence  of  natural  selection  is  differential  reproduc- 
tion. Thus,  many  factors  in  addition  to  survival  may  be 
significant.  Natural  selection  is  the  mechanism  through  which 
adaptation  is  achieved,  for  the  better  adapted  individuals  leave 
proportionately  more  offspring.  The  concept  of  natural  selection 
as  a  "struggle  for  existence"  or  "the  survival  of  the  fittest," 
though  correct  in  many  cases,  is  incomplete,  since  cooperative  be- 
havior or  even  altruism  may  also  be  developed  by  natural  selection 
if  they  contribute  to  reproductive  fitness.  The  efficacy  of  selection 
can  be  demonstrated  in  domesticated  species  as  well  as  in  natural 
populations.  Perhaps  the  most  unusual  example  was  the  work  on 
the  Baldwin  effect,  which  demonstrated  that  an  apparently 
Lamarckian  change  could  be  explained  within  the  existing  theoret- 
ical framework.  Selection  can  be  effective  only  in  heterozygous 
populations,  and  is  thus  without  effect  on  environmental  varia- 
tion. Selection  against  dominant  genes  will  be  considerably  more 
successful  than  against  recessives,  since  the  recessives  in  the  heter- 
ozygous condition  are  not  exposed  to  selection.  Ordinarily,  selec- 
tion pressures  and  mutation  pressures  are  opposed,  and  an  equi- 
librium between  the  origin  of  new  genes  through  mutation  and 
their  elimination  by  selection  is  achieved. 

SUGGESTED  READING 

Darwin,  C,  1872.  The  origin  of  species.  New  York:  Mentor  Books  (1958). 
Dobzhansky,   Th.,    1951.   Genetics  and  the  origin  of  species,   3d  ed.  New  York: 

Columbia  University  Press. 
Fisher,  R.  A.,   1930.  The  genetical  theory  of  natural  selection.  Oxford:  Clarendon 

Press.  (Also  Dover,  New  York.) 
Lerner,  I.  M.,  1958.  The  genetic  basis  of  selection.  New  York:  Wiley. 
,  1959.  "The  concept  of  natural  selection:  a  centennial  view,"  Proc.  Amer. 

Philosophical  Society,  103(2)  :173-182. 
Muller,  H.  J.,  1949.  "The  Darwinian  and  modern  conceptions  of  natural  selection," 

Proc.  Amer.  Philosophical  Society,  93(6)  :459-470. 
Schmalhausen,  I.  I.,  1949.  Factors  of  evolution.  The  theory  of  stabilizing  selection. 

(I.  Dordick,  tr.).  Philadelphia:  Blakiston. 
Sheppard,  P.  M.,  1958.  Natural  selection  and  heredity.  London:  Hutchinson. 


CHAPTER 


26 


Polymorphism 


If  natural  selection  constantly  causes  the  elimination  of 
the  less  fit,  in  time  a  population  might  be  expected  to  consist 
solely  of  the  best  adapted  type.  In  reality,  such  a  situation  seldom 
if  ever  exists,  for  despite  the  constant  pressure  of  natural  selec- 
tion, wild  populations  continue  to  have  considerable  genetic  vari- 
ability, a  fact  already  discussed  in  an  earlier  chapter.  Now  we 
must  consider  in  more  detail  how  this  variability  is  maintained. 

A  population  is  said  to  be  polymorphic  when  two  or 
more  distinct  types  of  individuals  coexist  in  the  same  breeding 
population.  Ford  has  limited  this  definition  further  by  saying  that 
the  forms  must  exist  in  such  proportions  that  the  rarest  is  not 
being  retained  in  the  population  merely  by  recurrent  mutation. 
However,  this  added  restriction  is  not  particularly  useful,  for  it 
presupposes  a  knowledge  of  the  mutation  rates  in  natural  popula- 
tions that  is  rarely  available,  and  it  cannot  easily  be  applied  except 
by  inference.  Polymorphism  is  used  with  respect  to  what  we  have 
earlier  called  discontinuous  traits  rather  than  for  continuous  varia- 
tion. These  traits  may  be  morphological,  in  which  case  they  are 
generally  controlled  by  two  or  more  alleles  of  a  gene  of  major 
effect,  and  therefore  present  no  difficulty  in  classification.  They 
may  also  be  chromosomal;  the  various  inversion  types  in  Dro- 
sophila  pseudoobscura  mentioned  earlier  represent  a  case  of  chro- 
mosomal polymorphism.  Furthermore,  human  populations  are  not 
only  polymorphic  for  many  morphological  traits,  but  they  are  also 
polymorphic  for  the  blood  groups.  Thus,  whether  polymorphism 
is  open  to  study  depends  to  some  extent  on  whether  suitable 

249 


250  •  THE    MECHANISM    OF   EVOLUTION 

methods  for  its  detection  have  been  devised.  Chromosomal  and  blood  group  dif- 
ferences are  clearly  discontinuous,  but  they  became  subjects  of  research  only  after 
cytological  and  serological  techniques  for  their  detection  had  been  developed. 

The  definition  is  intended  to  exclude  such  differences  as  are  observed 
between  geographical  races.  The  differences  between  members  of  the  same  species 
that  belong  to  different  breeding  populations  living  in  separate  areas  are  said  to 
be  polytypic.  Different  races  of  birds  may  overwinter  in  the  same  region  and  thus 
coexist  for  a  time,  but  this  situation  cannot  be  termed  polymorphism,  for  the 
races  are  still  members  of  separate  breeding  populations.  More  will  be  said  later 
about  the  origin  of  polytypic  differences  in  races,  but  at  this  point  we  shall  con- 
centrate on  polymorphism. 

In  the  chapter  on  selection  we  have  already  seen  that  an  equilibrium 
may  be  established  between  mutation  pressure  and  selection  pressure.  Thus  the 
polymorphism  observed  in  a  population  may  be  due  simply  to  the  balance  be- 
tween the  forces  of  mutation  and  selection.  Furthermore,  the  Hardy- Weinberg 
equilibrium  is  established  when  the  various  genotypes  all  have  the  same  selective 
value  or  are  adaptively  neutral.  Proof  of  adaptive  neutrality  is  virtually  impos- 
sible since  a  demonstration  that  no  selective  advantage  exists  under  one  set  of 
genetic  and  environmental  conditions  is  no  proof  that  it  might  not  exist  under 
somewhat  different  circumstances.  The  possible  variations  in  conditions  being 
almost  limitless,  pursuit  of  adaptive  neutrality  is  like  chasing  a  will  o'  the  wisp. 
Nevertheless,  it  remains  a  possibility  not  to  be  ignored,  especially  since  the  genes 
may  be  neutral  except  under  quite  specific  conditions.  However,  many  cases  of 
polymorphism  are  adaptive  and  clearly  involve  more  than  these  relatively  simple 
types  of  equilibria.  For  this  reason  polymorphism  has  assumed  a  significant  place 
in  evolutionary  studies. 

Transient  Polymorphism 

Two  additional  types  of  polymorphism  have  been  identified,  transient 
and  balanced.  Transient  polymorphism  exists  during  the  period  when  a  new  or 
previously  rare  mutant  becomes  advantageous  and  spreads  through  the  popula- 
tion. During  its  spread,  an  obvious  but  transient  polymorphism  will  exist.  It  is 
transient  because  the  new. form  will  eventually  (except  for  mutation)  replace  the 
old.  Balanced  polymorphism  exists  when selection .actively  maintains  more  than 
one  type  in  a  population,  A  variety  of  types  of  balanced  polymorphism  has 
been  discovered.  Because  of  their  very  nature,  balanced  polymorphisms  will  be 
more  common  than  examples  of  transient  polymorphism. 

The  most  carefully  studied  case  of  transient  polymorphism  is  the  phe- 
nomenon known  as  industrial  melanism,  which  has  been  observed  in  at  least 
70  species  of  moths  in  England  and  on  the  continent  of  Europe.  Although  other- 
wise they  may  be  quite  different,  all  of  these  moths  normally  rest  in  exposed 


POLYMORPHISM  •  251 


places,  depending  for  protection  on  their  cryptic  coloration,  a  mottled  pattern 
that  blends  in  with  a  background  of  bark  or  lichen.  The  industrial  revolution  of 
the  past  century  and  a  half  has  had  a  profound  effect  on  the  countryside  in 
industrial  regions.  The  smoke  and  soot  from  thousands  of  chimneys  have  coated 
trees  and  shrubs  for  miles  around.  As  a  consequence  the  background  on  which 
the  moths  now  must  rest  in  industrial  areas  is  much  darker  than  it  was  over  a 
century  ago.  A  remarkable  change  in  these  species  has  led  to  the  replacement  of 
the  typical  mottled  forms  by  much  darker  melanic  forms  in  the  industrial  areas. 
In  some  species   (for  example,  the  peppered  moth,  Bis  ton  betularia)   the  fre- 


Fig.  26-1.     Left:   dark  and  light  forms  of  the  peppered  moth   (Bis ton  betularia) 

on  the  trunk  of  an  oak  at  the  industrial  city  of  Birmingham,  England.    Right: 

dark  and  light  forms  of  the  peppered  moth  on  the  lichen-coated  trunk  of  an  oak 

in  an  unpolluted  region.    (Courtesy  of  Kettlewell.) 

quency  of  the  melanic  types  has  reached  over  95  percent  in  many  populations. 
Kettlewell  has  shown  that  in  industrial  regions  the  melanic  type  is  much  less 
likely  to  be  taken  by  birds  than  the  typical  mottled  moths,  but  that  in  unpolluted 
country  the  melanic  form  is  quite  conspicuous  and  is  subject  to  heavier  predation 
by  birds  than  are  moths  with  the  typical  pattern  (see  Fig.  26-1).  There  are  also 
indications  that  the  melanic  moths  may  differ  in  viability  or  behavior  from  the 
typical  form. 

In  virtually  all  of  the  species  the  transition  has  been  due  to  the  increase 
in  frequency  of  dominant  mutant  genes  for  melanism  even  though  recessive 
mutants  and  systems  of  multiple  factors  are  also  known  to  cause  increased 
melanin  production  in  at  least  some  of  these  species.  Since  the  various  kinds  of 


252  •  THE    MECHANISM    OF    EVOLUTION 

black  moths  were  all  rather  rare  prior  to  the  industrial  revolution,  it  is  quite 
clear  that  natural  selection  has  operated  specifically  to  bring  about  this  pheno- 
typic  transition  through  the  dominant  mutants  rather  than  through  some  other 
genetic  mechanism.  Although  other  reasons  for  the  utilization  of  dominants  have 
been  suggested,  the  most  obvious  was  given  many  years  ago  by  Haldane,  who 
showed  that  in  a  randomly  mating  population  a  rare  dominant  will  increase  in 
frequency  when  favored  by  selection  much  more  rapidly  than  will  a  rare  recessive 
or  a  rare  polygenic  system.  The  reasons  for  this  fact  are  quite  simple.  All  of  the 
dominant  mutants  are  exposed  to  selection  and  hence  when  selection  pressure 
shifts  to  favor  the  dominants,  half  of  their  progeny  will  carry  and  express  the 
dominant  in  the  next  generation  and  will  again  be  favored  by  natural  selection. 
Rare  recessive  individuals,  though  also  favored  by  selection  because  of  their 
phenotype,  will  seldom  leave  progeny  like  themselves  since  most  of  their  matings 
will  be  with  wild-type  individuals,  and  the  favored  recessive  mutant  will  be 
submerged  in  the  heterozygous  condition  in  the  population  until  by  chance  in 
future  generations  two  recessives  again  combine  in  a  single  individual.  Selection 
will  ordinarily  work  even  less  effectively  to  increase  the  frequency  of  rare  favor- 
able polygenic  systems,  since  they  are  constantly  being  broken  up  by  genetic  re- 
combination. Thus,  it  is  not  at  all  surprising  that  even  though  various  genetic 
mechanisms  causing  melanism  must  have  been  available  in  these  species,  the  one 
almost  invariably  selected  was  the  dominant  mutant. 

Although  industrial  melanism  in  moths  is  probably  the  most  closely 
studied  case  of  adaptive  polymorphism  involving  dominant  mutant  genes,  many 
other  examples  of  polymorphism  involving  dominants  to  the  wild  type  can  be 
cited.  Melanism  in  the  hamster  (Cricetus  crzcetus),  color  patterns  in  the  grouse 
locust  (Apotettix  eurycephalus),  in  the  platyfish  (Platypoecilus  maculatus),  in 
ladybird  beetles  (Coccznellzdae),  and  in  frogs  {burnsi  and  kandiyohi  mutants  in 
Rana  pipiens)  are  all  controlled  by  dominant  genes  and  have  relatively  high 
frequencies  in  natural  populations.  In  domesticated  plants  such  as  barley,  oats, 
wheat,  flax,  cotton,  cabbage,  and  tomatoes  many  cases  of  disease  resistance  con- 
trolled by  simple  dominant  mutations  can  be  cited.  Furthermore,  resistance  to 
subtertian  malaria  in  man  has  been  shown  to  be  increased  in  individuals  hetero- 
zygous for  the  sickle  cell  gene.  All  of  these  examples — and  more  could  be  cited 
— suggest  that  dominant  mutations  may  play  a  significant  role  not  only  in  poly- 
morphism but  in  evolution  as  well. 

The  Origin  of  Dominance 

Thus  far,  we  have  taken  dominance  and  recessiveness  more  or  less  for 
granted  although  we  have  discussed  the  fact  that  dominance  is  not  exclusively  a 
property  of  a  particular  gene,  but  may  be  modified  by  the  rest  of  the  genotype 
and  by  both  the  internal  and  external  environment  in  which  the  gene  functions. 


POLYMORPHISM  •  253 

At  this  point  it  seems  advisable  to  raise  the  question  of  the  origin  of  dominance. 
Several  hypotheses  have  been  advanced,  and  it  seems  likely  that  no  one  theory  is 
correct  and  the  others  wrong,  but  rather  that  each  contains  some  elements  of 
truth. 

Bateson  and  Punnett  were  the  first  to  suggest  a  theory  of  dominance 
when  they  proposed  that  the  recessive  condition  was  due  to  the  absence  of  the 
dominant.  This  simple  presence-absence  concept  became  untenable  after  the  dis- 
covery of  dominant  effects  due  to  deficiencies,  of  reverse  mutations  from  reces- 
sive to  dominant,  and  of  multiple  alleles. 

Fisher  pointed  out  that  the  great  majority  of  mutants  that  occur  are 
deleterious  and  are  recessive  to  the  "normal"  or  "wild-type"  alleles  found  in 
natural  populations,  and  he  thus  framed  the  question  in  terms  of  the  origin  of 
dominance  of  wild-type  genes.  He  further  noted  that  mutations  are  recurrent  and 
frequent  enough  so  that  a  given  mutant  will  be  regularly  reintroduced  into  a 
population  even  though  it  is  deleterious.  He  assumed  that  the  very  first  time  a 
particular  mutation  occurs,  the  heterozygote  will  be  phenotypically  intermediate 
between  the  two  homozygotes.  Dominance  will  then  arise  as  the  result  of  the 
selection  of  modifying  factors  at  other  loci  that  push  the  expression  of  the  inter- 
mediate heterozygote  toward  that  of  the  homozygous  wild  type. 

Several  difficulties  in  this  theory  should  be  pointed  out.  The  assumption 
of  an  initially  intermediate  heterozygote  is  in  a  sense  gratuitous,  for  it  is  actually 
part  of  what  must  be  proven.  Furthermore,  the  theory  offers  no  adequate  expla- 
nation for  the  appearance  of  the  occasional  recurrent  deleterious  mutant  that  is 
dominant  to  the  wild  type.  Wright  has  also  estimated  that  heterozygotes  will  be 
so  infrequent  and  the  selective  advantage  so  slight  that  the  selection  pressures 
will  be  too  small  to  be  a  controlling  factor  in  the  fixation  of  modifiers.  In  addi- 
tion, the  modifiers  will  have  other  primary  effects  of  their  own,  and  their  ulti- 
mate frequency  will  depend  more  on  the  action  of  selection  with  respect  to  these 
primary  effects  than  it  will  on  their  effects  on  the  dominance  of  some  other  gene. 

As  an  alternative  to  Fisher's  theory  of  modifiers  Wright  suggested  a 
physiological  theory  of  dominance.  He  noted  that  the  normal  or  wild-type  genes 
are  functional,  but  deleterious  mutants  represent  a  partial  or  complete  inactiva- 
tion  of  the  gene.  Dominance  then  results  because  the  wild-type  allele,  which  is 
active,  will  be  expressed  in  the  presence  of  the  deleterious  mutant,  which  is  not. 
The  genes  are  presumed  to  control  the  formation  of  enzymes,  which  catalyze 
chemical  reactions  in  living  things.  The  rate  of  these  enzymatic  reactions  depends 
on  both  the  concentration  of  the  enzyme  and  that  of  the  substrate.  If  a  single 
normal  gene  in  a  heterozygote  produces  enough  enzyme  for  a  reaction  to  proceed 
at  the  maximum  rate  possible,  the  heterozygote  will  resemble  the  homozygote, 
and  dominance  will  be  complete.  If,  on  the  other  hand,  it  does  not  produce 
enough  enzyme,  dominance  will  be  incomplete,  but  the  greater  the  activity  of  the 
gene,  the  more  the  heterozygote  will  resemble  the  homozygote. 


254  •  THE    MECHANISM    OF    EVOLUTION 

Haldane  proposed  that  dominance  resulted  from  the  selection  of  the 
more  efficient  wild-type  alleles  from  among  a  group  of  different  wild-type  alleles 
or  isoalleles.  Since  individuals  heterozygous  for  the  more  active  allele  would  be 
more  like  the  normal  homozygote,  they  would  have  a  selective  advantage  in 
heterozygotes,  and  the  more  active  allele  would  be  favored  by  selection  over  the 
less  active  type.  Thus  he  argued  that  selection  would  favor  the  allele  that  had  a 
safety  factor  of  at  least  two  in  enzyme  production  so  that  a  single  gene  could 
perform  the  task  ordinarily  done  by  two.  This  theory,  like  Wright's,  is  essentially 
a  physiological  theory  of  dominance. 

A  final  theory,  developed  by  Plunkett  and  Muller,  again  involves  the 
selection  of  modifiers.  Unlike  Fisher's  idea,  however,  selection  is  directed,  not 
primarily  at  the  infrequent  heterozygotes,  but  at  the  wild-type  homozygotes. 
Those  modifying  factors  are  selected  that  tend  to  stabilize  the  wild-type  pheno- 
type  under  all  sorts  of  environmental  and  genetic  stresses.  Under  this  hypothesis, 
modifiers  are  selected  not  just  for  their  ability  to  suppress  the  harmful  effects  of 
an  occasional  deleterious  mutant,  but  rather  to  build  up  a  safety  factor  for  the 
wild  type. 

From  the  wealth  of  theories  it  is  clear  that  the  question  of  the  origin  of 
dominance  has  not  yet  been  finally  resolved.  Experimental  evidence  can  be  cited 
in  support  of  both  the  physiological  and  modifier  theories.  There  is  no  question, 
for  example,  that  dominance  can  be  shifted  by  the  selection  of  suitable  modifiers. 
Nevertheless,  it  is  also  true  that  different  wild-type  alleles  may  show  different 
degrees  of  dominance  in  heterozygotes.  The  theories  are  not  mutually  exclusive, 
for  it  is  quite  conceivable  that  mutants  may  occur  that  are  favorable  and  domi- 
nant from  the  outset  and  are  immediately  favored  by  selection.  However,  if  such 
mutants  are  not  available,  selection  may  be  forced  to  work  with  the  genetic  mate- 
rials at  hand  to  increase  the  dominance  of  existing  mutants  through  modifiers  at 
other  loci. 

Balanced  Polymorphism 

Balanced  polymorphism  may  arise  in  a  number  of  different  ways.  If  the 
rarer  form  were  always  at  a  selective  advantage,  adaptive  values  would  change  as 
frequencies  changed.  A  rare  form  favored  by  selection  would  lose  this  selective 
advantage  as  it  became  more  common,  until  at  high  frequencies  it  would  be  at  a 
disadvantage.  In  this  way  selection  would  tend  to  damp  any  oscillations  in  gene 
frequency  before  they  led  to  the  extinction  of  one  allele,  and  a  balanced  situation 
would  be  maintained.  Such  a  situation  might  arise  as  a  result  of  the  feeding 
habits  of  predators  that  tend  to  take  the  common  forms  of  their  polymorphic 
prey  but  overlook  the  rare  ones. 

In  the  twin-spot  ladybird  beetle  (Adalia  bipunctatOi)  changing  selection 
pressures  of  a  somewhat  different  kind  are  responsible  for  still  another  type  of 


POLYMORPHISM   •  255 

equilibrium.  The  red  phase  increases  in  relative  frequency  during  the  winter,  but 
the  black  phase  increases  during  the  summer.  As  a  result  of  the  seasonal  shifts  in 
adaptive  value,  neither  type  is  eliminated.  Similar  seasonal  shifts  in  the  frequency 
of  inversion  types  in  Drosophila  pseudoobscura  indicate  that  balanced  poly- 
morphism is  a  device  by  which  this  species,  too,  adapts  to  seasonal  changes. 
Seasonal  polymorphism  is  more  apt  to  be  observed  in  species  with  a  short  genera- 
tion length. 

A  rather  unusual  type  of  polymorphism  is  exemplified  by  the  T  locus  in 
mice.  A  number  of  distinct  alleles  have  been  found  in  different  wild  populations 
that  in  the  homozygous  condition  cause  sterility  or  even  lethality  but  have  no 
visible  effect  on  the  phenotype  of  heterozygotes.  Mendelian  segregation  in 
heterozygous  females  is  normal,  so  that  eggs  bearing  mutant  and  normal  genes 
are  produced  in  equal  numbers.  However,  in  heterozygous  males,  segregation  is 
highly  abnormal,  for  up  to  95  percent  of  the  sperm  cells  carry  the  deleterious 
mutant.  Under  these  circumstances,  the  increase  in  frequency  of  the  mutant  that 
would  otherwise  occur  is  checked  or  held  in  balance  by  the  lethal  or  sterile  effects 
of  the  gene.  Comparable  examples  have  been  described  in  Drosophila  under  the 
term  "meiotic  drive."  Many  questions  remain  to  be  answered  about  what  appear 
to  be  most  peculiar  and  anomalous  situations. 

Any  system  whereby  mating  between  individuals  of  unlike  genotype  is 
encouraged  or  enforced  leads  to  the  establishment  of  a  stable  polymorphism.  In- 
compatibility systems  in  plants  are  a  case  in  point.  Some  species  such  as  red 
clover  (Trifolium  pratense)  have  a  series  of  multiple  self -sterility  alleles,  Slf  S2, 
5*3,  S±,  etc.  Pollen  that  carries  any  particular  allele  will  fail  to  fertilize  the  ovules 
of  any  plant  carrying  the  same  allele.  Thus  St  pollen  will  successfully  fertilize 
ovules  in  S2S3,  S2S4,  and  S3S4  plants  but  not  in  S^z,-  S1S3,  or  StS4  plants.  Self- 
fertilization  is  therefore  impossible,  and  furthermore  no  homozygotes  can  be 
formed. 

The  Pin-Thrum  situation  in  the  primrose  {Primula  vulgaris)  is  com- 
parable but  differs  in  some  respects.  Pin  flowers  have  a  long  style  with  the  stigma 
at  the  mouth  of  the  corolla  tube  of  the  flowers  and  the  anthers  half-way  down 
the  tube.  In  Thrum  flowers  the  positions  of  anthers  and  stigma  are  reversed  as 
compared  to  Pin.  This  difference  ordinarily  behaves  as  if  controlled  by  a  single 
locus,  with  Pin  being  the  homozygous  recessive  (pp)  and  Thrum  the  hetero- 
zygote  (Pp).  The  pollen  tube  formed  by  Pin  pollen  grows  only  very  slowly  on 
Pin,  but  Thrum  pollen  on  a  Thrum  stigma  forms  no  pollen  tube  at  all.  Since 
Thrum  is  a  heterozygote,  its  pollen  is  of  two  types.  Therefore,  the  pollen  be- 
havior must  be  determined,  not  by  the  genotype  of  the  pollen  itself  as  with  the 
self-sterility  alleles,  but  by  the  genotype  of  the  Thrum  parent,  for  p  pollen  from 
a  Pin  plant  will  grow  down  the  style  of  a  Thrum  (Pp)  plant,  but  genetically 
similar  p  pollen  from  a  Thrum  plant  will  not. 

In  animals,  nonrandom  mating  has  occasionally  been  reported  in  which 


256  •  THE    MECHANISM    OF    EVOLUTION 

unlike  individuals  are  more  apt  to  mate  than  individuals  of  like  genotype.  If 
negative  assortative  mating  of  this  kind  actually  does  occur,  it  too  would  result  in 
balanced  polymorphism,  for  individuals  of  the  rarer  type  would  have  a  greater 
likelihood  of  obtaining  mates.  This  case  actually  represents  still  another  way  in 
which  selection  intensity  would  be  related  to  gene  frequency. 

Although  this  category  is  seldom  included  in  discussions  of  polymorph- 
ism, it  is  worth  pointing  out  that  any  species  with  separate  sexes  is  polymorphic 
in  every  sense  of  the  word.  In  most  cases  this  polymorphism  is  chromosomal  as 
well  as  phenotypic,  and  cross  fertilization  is  mandatory.  In  addition  to  the 
primary  differences  between  the  sexes,  there  are  many  secondary  sexual  charac- 
ters. The  adaptive  value  of  these  traits  in  many  cases  seems  quite  apparent,  but 
much  remains  to  be  learned  about  these  adaptive  values,  their  mode  of  origin  by 
selection,  and  the  genetic  mechanisms  controlling  them. 


Heterosis  and  Polymorphism 

The  final  mechanism  of  balanced  polymorphism  to  be  discussed  is  the 
situation  in  which  the  heterozygote  is  more  fit  than  either  homozygote.  In  other 
words,  heterosis  may  also  serve  as  a  means  of  maintaining  balanced  polymorph- 
ism. The  most  extreme  case  of  this  sort  is  a  balanced  lethal  system.  If  linkage  is 
close  or  crossing  over  is  in  some  way  suppressed,  only  Ab/aB  progeny  will  re- 
sult from  Ab/aB  heterozygous  parents,  for  the  Ab/Ab  and  aB/aB  homozygotes 
will  die  owing  to  the  homozygous  recessive  lethals  (bb  or  aa).  Individuals  of 
the  Ab/aB  type  will  breed  true  in  spite  of  being  heterozygous. 

Overdominance  will  also  lead  to  a  balanced  heterozygous  system.  In 
this  case  only  a  single  locus  need  be  involved,  and  the  homozygotes  may  be  only 
slightly  inferior  to  the  heterozygote.  When  the  heterozygote  (Aa)  is  superior, 
selection,  rather  than  tending  toward  homozygosity  for  a  favored  allele,  will 
favor  the  heterozygotes,  and  hence  will  produce  a  stable  equilibrium  at  the  gene 
frequencies  that  confer  optimum  fitness  on  the  entire  population.  These  fre- 
quencies are  determined  by  the  relative  fitness  of  the  two  homozygotes.  If  the 
fitness  of  Aa  is  set  equal  to  1,  of  A  A  equal  to  (1  —  sx),  and  of  aa  equal  to 
(1  —  j-2),  then 


*V        1  -  sxf  -  stf 
and  at  equilibrium  Ap  =  0  and  s\p  =  s2q 

Solving  this  equation, 

A  J"2 


J-l  +    J-2 


POLYMORPHISM 

For  example, 

if  Aa  =  1 

AA  =  1 

-  ji  =  .8 

C'l  =    -2) 

aa  —  1 

-  j-2  =  .4 

(i-2  =    -6) 

Then 

A 

6    ,  =  -75 

257 


The  best  example  of  single  gene  heterosis  responsible  for  balanced 
polymorphism  comes  from  man.  The  sickle  cell  gene  (Hbs)  produces  an  ab- 
normal hemoglobin  and  in  homozygous  condition  causes  sickle  cell  anemia,  a 
debilitating  disease  that  is  usually  fatal.  This  gene  has  a  surprisingly  high  fre- 
quency in  some  parts  of  the  world.  In  these  areas  malaria  is  endemic,  and  it  has 
been  found  that  the  heterozygotes  (Jibs/Hba)  for  the  sickle  cell  gene  are  signifi- 
cantly more  resistant  to  subtertian  malaria  than  are  the  homozygotes  (Hba/Hba) 
for  normal  adult  hemoglobin.  Thus  where  malaria  is  prevalent,  the  heterozygotes 
are  better  adapted  than  the  homozygotes,  which  are  apt  to  die  either  from  anemia 
on  the  one  hand  (Hbs/Hbs)  or  malaria  on  the  other  (Hba/Hba). 

Probably  the  most  thoroughly  studied  case  of  heterozygote  superiority 
is  that  of  inversion  heterozygotes  in  Drosophila.  In  some  species  of  Drosophila 
(for  example,  D.  pseudoobscura,  D.  persimilis,  D.  miranda,  D.  robusta,  and  D. 
willistorii)  two  or  more  inversions  may  occur  with  high  frequency  within  a 
single  breeding  population.  The  seasonal  shifts  in  frequency  of  inversion  types 
have  already  been  mentioned,  but  even  more  significant  is  the  fact  that  the  inver- 
sion heterozygotes  show  hybrid  vigor  or  superior  fitness  as  compared  to  the  in- 
version homozygotes  even  though  their  external  appearances  are  similar.  The 
implication  is  clear  that  the  different  inversion  types  must  differ  to  some  extent 
in  their  gene  contents.  Since  crossing  over  is  restricted  in  inversion  heterozygotes, 
the  development  of  these  differences  is  not  surprising.  This  is  not  to  suggest, 
however,  that  all  chromosomes  of,  say,  the  Standard  type  in  a  breeding  popula- 
tion of  D.  pseudoobscura  have  the  same  gene  contents,  but  merely  that  two 
Standard  chromosomes  from  the  same  population  will  generally  be  more  alike 
than  will  a  Standard-  and  an  Arrowhead-type  chromosome  drawn  from  the  same 
population.  Since  the  block  of  chromatin  within  an  inversion  will  be  isolated 
from  recombination  with  other  inversions,  the  gene  complex  within  an  inversion 
will  be  subject  to  selection  as  a  unit.  These  gene  complexes  can  thus  be  expected 
to  differ  from  each  other  in  both  gene  contents  and  adaptive  value.  Furthermore, 
it  has  been  postulated  that  selection  will  also  operate  to  favor  those  combinations 
of  genes  in  each  inversion  type  that  confer  maximum  heterosis  or  fitness  when  in 
heterozygous  combination  with  another  inversion,  since  inversion  heterozygotes 
are  ordinarily  more  common  than  inversion  homozygotes.  Thus,  in  addition  to 
its  adaptive  value  as  a  homozygote  each  inversion  type  may  have  an  adaptive 


258  •  THE    MECHANISM    OF   EVOLUTION 

value  as  a  heterozygote,  or  will  be  "coadapted"  to  the  other  gene  complexes  in 
the  population. 

One  additional  observation  about  these  inversion  heterozygotes  should 
be  noted.  In  general,  the  heterozygotes  are  phenotypically  more  stable  or  show 
less  variation  under  environmental  stress  than  do  the  corresponding  homozygotes. 
Furthermore,  a  heterozygous  population  is  better  able  to  adapt  to  changing  en- 
vironmental conditions  without  major  disruptions  than  is  a  relatively  homozygous 
population.  These  two  concepts,  in  some  ways  related,  have  been  widely  dis- 
cussed under  the  terms  "developmental  homeostasis"  and  "genetic  homeostasis" 
respectively. 

That  the  different  inversions  do  differ  in  adaptive  value  is  indicated  by 
their  seasonal  and  altitudinal  shifts  in  frequency.  In  population  studies  in  Cali- 
fornia, for  example,  the  Standard  type  in  D.  pseudoobscura  increased  in  fre- 
quency as  the  weather  became  warmer,  reaching  a  maximum  during  the  hot 
summer  months.  Populations  sampled  at  different  altitudes  formed  a  cline  with 
Standard  having  a  low  frequency  at  high  altitudes  and  increasing  in  frequency 
with  lower  elevation.  Since  altitude  also  provides  a  temperature  gradient,  the 
Standard  gene  complex  in  this  region  appears  to  be  better  adapted  to  warmer 
temperatures  than  the  other  inversions  in  these  populations.  Here,  as  demon- 
strated previously,  the  relative  frequencies  in  this  balanced  polymorphic  system 
will  be  determined  by  the  relationship  between  the  selection  coefficients  of  the 
homozygous  types. 

Samples  taken  over  the  wide  geographical  range  of  a  species  may  also 
show  shifts  in  the  frequency  and  kinds  of  the  different  third-chromosome  inver- 
sions. These  differences  undoubtedly  reflect  changing  adaptive  requirements 
under  different  ecological  conditions,  but  they  may  also  reflect  historical  events, 
in  the  sense  that  different  chromosomal  mutations  may  have  occurred  in  different 
parts  of  the  range.  Since  selection  must  operate  within  the  framework  of  the 
available  variability,  some  of  the  geographic  variation  in  inversion  types  may 
have  arisen  in  this  way. 

The  amount  of  inversion  heterozygosity  has  been  found  to  vary  greatly, 
usually  being  maximal  toward  the  center  of  the  range  of  a  species  and  decreasing 
toward  the  periphery.  One  theory  proposes  that  chromosomal  polymorphism  per- 
mits the  species  to  exploit  a  greater  variety  of  ecological  niches  than  would  other- 
wise be  open  to  it.  Thus,  at  the  center  of  the  range  the  species  is  presumed  to 
be  highly  successful,  exploiting  a  number  of  different  niches,  but  at  the  limits  of 
the  range  the  environment  is  marginal  for  the  species  and  a  minimal  number  of 
niches  are  habitable. 

Another  hypothesis  is  that  the  primary  function  of  inversion  hetero- 
zygosity in  natural  populations  is  related  to  its  effects  on  recombination.  In  the 
central  populations,  with  a  high  frequency  of  inversion  heterozygosity,  the 
amount  of  possible  genetic  recombination  will  be  considerably  restricted.  Selec- 


POLYMORPHISM  •  259 

tion  will  tend  to  favor  heterozygotes  with  superior  general  vigor,  and  adaptation 
will  be  achieved  through  heterosis.  This  type  of  adjustment  is  only  feasible  in 
large  populations,  for  it  is  made  at  the  expense  of  the  production  of  homozygotes 
of  low  fitness.  Any  device,  such  as  an  inversion,  that  would  tend  to  reduce  the 
frequency  with  which  relatively  unfit  homozygotes  are  formed  will  have  an  im- 
mediate selective  value  because  it  will  minimize  the  cost  of  maintaining  heterosis 
in  the  population.  When  adaptation  via  heterosis  occurs,  the  population  can  meet 
rather  drastic  environmental  changes  with  relatively  minor  adjustments  in  its 
heterotic  genetic  system;  it  is  said  to  be  "heterotically  buffered."  However,  such 
a  system  imposes  a  considerable  limitation  on  the  possibilities  for  future  evolu- 
tionary change. 

On  the  other  hand,  in  marginal  populations,  small  in  numbers  and  rela- 
tively isolated,  inversion  heterozygosity  is  low  and  genetic  recombination 
relatively  unrestricted.  Under  these  circumstances  selection  will  tend  toward  the 
ultimate  fixation  of  those  genes  conferring  superior  fitness.  It  is  in  these  popula- 
tions, it  is  argued,  that  the  evolutionary  changes  occur  that  lead  to  genetic  diver- 
gence and  ultimately  to  the  formation  of  new  subspecies  and  species. 

Although  a  great  deal  of  very  fascinating  work  has  been  done  on 
chromosomal  polymorphism  in  Drosophila,  it  seems  likely  that  there  is  still  much 
to  be  learned.  For  example,  why  should  inversion  heterozygosity  be  so  common 
in  some  species  of  the  genus  Drosophila  but  rare  or  absent  in  other  species  such 
as  D.  melanogaster  and  D.  virilis,  which  are  widely  distributed  and  highly  suc- 
cessful in  exploiting  a  variety  of  ecological  niches?  A  most  interesting  observa- 
tion made  some  years  ago  by  Dubinin  in  Russia  showed  that  the  frequency  of 
inversion  heterozygosity  in  D.  funebris  was  related  to  the  degree  of  industrializa- 
tion of  the  area  in  which  the  population  lived.  Thus,  populations  in  large  urban 
areas  showed  a  high  degree  of  inversion  heterozygosity,  but  the  frequency  de- 
clined in  suburban  and  small-town  populations  until  it  was  virtually  zero  in  rural 
districts.  This  difference  may  well  be  related  to  the  number  of  adaptive  niches 
available  in  urban  as  compared  to  rural  areas,  but  it  may  also  reflect  the  effect  of 
differences  in  population  size  of  the  flies  or  of  passive  transport  of  flies  into  the 
cities.  Only  further  study  can  resolve  these  questions. 

The  material  already  presented  should  suffice  to  illustrate  some  of  the 
complexities  related  to  polymorphism,  but  still  other  aspects  of  this  subject 
may  be  mentioned.  Many  instances  of  mimicry,  for  example,  also  involve  poly- 
morphism, sometimes  affecting  just  one  sex  and  not  the  other.  Environmental 
factors  may  also  induce  polymorphic  differences;  pupa  case  color  in  certain 
species  of  butterflies  is  related  to  the  type  of  background  on  which  chrysalis 
formation  occurs.  Green  pupae  are  more  common  on  the  green  leaves  of  plants 
whereas  brown  pupae  are  more  frequent  if  the  pupae  are  formed  on  the  brown 
stems.  These  differences  reflect  a  delicate  adjustment  between  the  genotype  and 
the  environment.  Still  other  polymorphisms  observed  in  the  field  may  be  due  to 


260  •  THE    MECHANISM    OF   EVOLUTION 

the  ability  of  the  individual  organism  to  change  its  color  to  match  its  back- 
ground, an  ability  fairly  common  in  the  animal  kingdom.  Tree  frogs  among  the 
amphibians,  the  chameleon  among  the  reptiles,  and  the  cuttlefish,  a  molluscan 
invertebrate,  are  familiar  examples  of  species  with  great  capacity  in  this  respect. 
In  man,  polymorphisms  of  many  kinds  may  be  observed,  but  their  sig- 
nificance is  usually  unknown.  In  the  past,  the  blood  groups  were  frequently  re- 
ferred to  as  adaptively  neutral  traits,  but  the  discovery  of  the  relation  between 
the  sickle  cell  gene  and  malarial  resistance,  and  between  other  blood  group  genes 
in  the  ABO  system  and  the  incidence  of  stomach  cancer  and  duodenal  ulcer  indi- 
cates that  this  is  a  hazardous  assumption.  Other  cases  present  problems  of 
particular  interest  and  importance.  Both  schizophrenia  and  diabetes  have  an  inci- 
dence in  human  populations  of  about  1  percent  despite  the  fact  that  the  repro- 
ductive rate  of  affected  persons  in  the  past  must  have  been  significantly  lower 
than  that  of  unaffected  individuals.  Since  an  underlying  genetic  basis  has  been 
demonstrated  for  both  illnesses,  the  high  frequency  of  diabetes  and  schizophrenia 
suggests  the  existence  of  balanced  polymorphism,  but  the  possible  mechanism 
remains  unknown.  The  study  of  polymorphism  has  been  an  exceptionally  fruitful 
area  of  research  for  students  of  variation  and  evolution,  and  these  and  many 
other  problems  suggest  that  it  will  continue  to  be  so  for  some  time  to  come. 


SUMMARY  <- 


A  polymorphic  population  contains  two  or  more  distinct 
types  of  individuals.  Not  only  genie  but  chromosomal  polymorph- 
isms have  been  discovered.  Polymorphism  may  result  from  the 
Hardy-Weinberg  equilibrium  or  from  the  balance  between  the 
opposing  forces  of  mutation  and  selection.  Of  even  greater  inter- 
est are  transient  and  balanced  polymorphism.  The  most  thoroughly 
studied  case  of  transient  polymorphism,  industrial  melanism,  has 
shown  that  in  industrial  regions  in  Europe,  the  light,  mottled 
pattern  of  many  moths  has  been  almost  completely  replaced  in  a 
matter  of  decades  by  a  darker,  melanic  form,  better  adapted  to 
the  new  background.  Numerous  examples  of  polymorphism  in- 
volving dominant  mutants  are  known,  and  there  are  various 
theories  of  the  origin  of  dominance.  Balanced  polymorphism  may 
be  due  to  a  number  of  conditions,  among  them  shifting  selection 
pressures  and  selection  favoring  the  heterozygotes  over  both 
homozygotes.  The  study  of  balanced  polymorphism  has  loomed 
large  in  recent  work  on  the  nature  and  origin  of  species,  and  it 
remains  a  fertile  field  for  research. 


POLYMORPHISM  •  26l 


SUGGESTED  READING 

Cold  Spring  Harbor  Symp.  Quant.  Biol.,  Vol.  20,  1955.  "Population  genetics."  Long 

Island  Biological  Assoc,  New  York. 
Dobzhansky,   Th.,    1951.   Genetics  and  the  origin   of  species,   3d  ed.   New  York: 

Columbia  University  Press. 
Sheppard,  P.  M.,  1958.  Natural  selection  and  heredity.  London:  Hutchinson. 


CHAPTER 


27 

Genetic  Drift 


Thus  far  in  our  discussions  of  the  genetics  of  populations 
we  have  been  making  the  implicit  assumption  that  the  populations 
were  infinitely  large.  In  actuality  natural  populations  are,  of 
course,  finite  in  size  and  may  be  quite  small.  JEven  when  the  total 
population  is  very  large,  if  it  is  divided  into  numerous  small,  iso- 
lated, breeding  populations,  the  dynamics  of  the  changes  in  gene 
frequency  will  be  determined  by  the  forces  operating  in  each 
small  population  independent  of  the  rest.  If  there  is  some  migra- 
tion between  the  different  breeding  populations,  the  evolutionary 
course  of  the  entire  species  will  be  tied  together  in  a  very  complex 
manner  that  depends  not  only  on  mutation  pressure  and  the  selec- 
tion pressures  within  and  between  populations,  but  also  on  the 
size  of  the  various  breeding  populations  and  on  the  amount  of 
migration  between  them.  We  have  already  considered  the  effects 
of  mutation  and  selection.  Now  we  must  discuss  the  effect  of 
population  size  on  an  isolated  population,  before  going  on  in  a 
later  chapter  to  treat  migration  or  gene  flow. 

The  total  number  of  individuals  in  a  species,  without 
reference  to  the  way  in  which  the  species  may  be  subdivided  into 
breeding  populations,  gives  little  indication  of  the  possible  effects 
of  population  size  on  gene  frequency  changes.  Similarly,  a  simple 
census  of  the  number  of  individuals  in  a  single  population  may 
not  be  a  true  index  of  the  effective  breeding  size  of  the  popula- 
tion. Some  species,  for  example,  undergo  drastic  periodic  seasonal 
fluctuations  in  numbers.  A  census  taken  in  the  fall  may  indicate  a 
size  in  the  hundreds  of  thousands  or  even  millions  for  an  insect 


262 


GENETIC    DRIFT  •  263 

population  in  the  temperate  zone.  However,  if  only  a  fraction  of  1  percent  of 
these  insects  survive  the  winter,  the  characteristics  of  this  population  will  largely 
be  determined  by  this  handful  of  survivors  rather  than  by  the  much  larger  num- 
ber at  the  population  peak. 

We  have  already  seen  in  the  discussion  of  the  Hardy-Weinberg  equilib- 
rium that  in  a  large,  randomly  mating  population,  in  which  there  is  no  mutation 
or  selection,  gene  frequencies  will  remain  constant.  However,  if  the  population 
is  small,  gene  frequencies  will  tend  to  fluctuate  purely  by  chance,  and  the  smaller 
the  population,  the  greater  the  fluctuations  are  apt  to  be.  These  random  changes 
in  gene  frequency  are  said  to  be  due  to  genetic  drift.  The  gene  frequencies  in  a 
small  population  will  continue  to  fluctuate  until  one  allele  is  lost  and  the  other 
fixed.  Subsequently,  the  population  will  remain  homozygous  unless  a  new  muta- 
tion appears. 

—  ^'"The  basis  for  genetic  drift  is  to  be  found  in  the  process  of  sampling. 
In  order  to  understand  the  relation  between  population  size  and  drift,  we  must 
understand  certain  elementary  principles  of  sampling.  If  the  gene  A  is  repre- 
sented by  a  black  marble  and  its  allele  a  by  a  white  one,  then  all  of  the  gametes 
produced  by  a  population  can  be  represented  by  a  large  bowl  full  of  marbles, 
with  the  black  marbles  representing  the  proportion  of  A  genes  in  the  gametes. 
Obviously  not  all  of  the  gametes  produced  will  go  to  form  the  next  generation, 
for  many,  especially  the  sperm,  will  not  take  part  in  fertilization,  and  many  of 
the  fertilized  eggs  will  not  survive  to  maturity.  Thus,  the  gametes  that  actually 
give  rise  to  the  next  generation  can  be  represented  by  a  handful  of  marbles  taken 
from  the  bowl.  If  there  are  equal  numbers  of  A  and  a  genes  in  the  gametes,  the 
gene  frequency  of  A  is  50  percent.  However,  in  a  handful  of  marbles  taken  at 
random,  it  is  unlikely  that  the  numbers  will  be  exactly  equal.  Similarly,  because 
of  the  random  nature  of  meiosis  and  fertilization,  the  numbers  of  dominant  and 
recessive  genes  may  not  be  equal.  The  principles  involved  in  estimating  how 
large  the  deviations  from  equality  may  be  are  much  the  same  as  those  used  in 
estimating  the  expected  numbers  of  heads  and  tails  with  a  tossed  coin.  If  you 
tossed  a  penny  four  times,  you  would  probably  not  be  surprised  if  you  got  three 
tails  and  one  head.  In  fact,  it  can  be  estimated  that  such  a  result  would  be  ex- 
pected 25  percent  of  the  time  when  four  tosses  are  made.  The  probabilities  for 
various  combinations  of  heads  and  tails  on  four  tosses  are  calculable  from  ex- 
pansion of  the  binomial  (a  +  &)4,  where  a  =  y2  =  the  probability  of  heads, 
and  b  =  ]/2  =  the  probability  of  tails.  The  complete  expansion  is : 


3  heads 

2  heads 

1  head 

composition  of  sample 

4  heads 

1  tail 

2  tails 

3  tails 

4  tails 

proportion  of  heads 

1 

.75 

.50 

.25 

0 

frequency 

a" 

4azb 

6a2b2 

4ab* 

b± 

probability  of  sample 

of  above  type 

% 

He 

% 

Vm 

He 

*- 


264  •  THE    MECHANISM    OF   EVOLUTION 

Thus,  less  than  half  the  time  (%6)  would  you  expect  to  get  equal  numbers  of 
heads  and  tails,  or  of  black  and  white  marbles,  or  of  dominant  and  recessive 
genes  in  samples  of  four  drawn  from  a  source  of  supply  in  which  each  type  has 
an  equal  frequency.  In  terms  of  gene  frequencies,  it  is  clear  that  there  is  a  sizable 
chance  that  the  frequency  of  A  will  shift  either  to  .75  or  .25  or  that  A  may 
become  either  fixed  or  lost  from  the  population. 

However,  if  you  tossed  a  penny  10,000  times,  you  would  be  very  sur- 
prised if  you  got  7500  tails  and  only  2500  heads,  and  rightly  so,  even  though 
the  ratio  of  heads  and  tails  is  the  same  as  for  3  tails  and  1  head.  Your  more  or 
less  instinctive  reaction  can  be  borne  out  statistically,  for  the  standard  error  of  a 

ratio  for  large  samples  equals    .  /   P  X  1      or  in  this  case    A  /    (°-30)  (0.50) 

\        n  \  10,000 

=  0.005.  Thus  with  10,000  tosses,  expectations  are  for  5000  heads,  with  a 
standard  error  of  50.  Since  the  chances  are  less  than  1  in  100,000  that  a  sample 
will  diverge  from  its  source  by  as  much  as  four  times  its  standard  error,  even  a 
ratio  of  5200  tails  to  4800  heads  would  be  extremely  improbable.  From  this  line 
of  reasoning,  it  should  be  clear  why  random  fluctuations  in  gene  frequency  tend 
to  be  larger,  the  smaller  the  sample  of  genes  that  gives  rise  to  the  next  generation. 
One  further  point  to  note  is  that  the  sample  of  genes  that  goes  to  form 
the  first  generation  will  then  in  its  turn  generate  the  new  supply  of  gametes  from 
which  the  genes  of  the  second  generation  will  be  drawn.  Therefore,  if  sampling 
fluctuations  have  resulted  in  frequencies  of  A  and  a  other  than  0.5,  the  sampling 
situation  is  likely  to  be  somewhat  different  in  the  next  generation  than  it  was  in 
the  preceding  one.  If,  for  example,  1  white  and  3  black  marbles  were  drawn  at 
random  from  a  bowl  containing  equal  numbers  of  black  and  white,  the  new  bowl 
of  marbles  from  which  the  next  sample  must  be  drawn  would  contain,  not  equal 
numbers  of  black  and  white,  but  %  black  and  only  y^  white.  Over  a  number  of 
generations,  sampling  fluctuations  may  have  a  cumulative  effect  and  gene  fre- 
quencies may  diverge  considerably  from  their  initial  frequencies,  hence  the  name 
"genetic  drift."  As  a  result  of  random  genetic  drift  a  new  mutant  may  occa- 
sionally spread  through  a  small  population  until  it  becomes  homozygous  or  fixed 
in  the  population,  but  more  often  random  drift  will  lead  to  the  loss  of  the  new 
allele  before  it  has  even  had  a  chance  to  spread. 


Effective  Size  of  Populations 

The  effects  of  genetic  drift  have  been  estimated  under  various  condi- 
tions, but  a  special  case  of  rather  general  interest  will  suffice  to  give  some  indica- 
tion of  the  relation  between  population  size  and  genetic  drift.  In  a  population  of 
moderate  size  with  equal  numbers  of  males  and  females  mating  at  random,  the 
rate  of  decay  of  the  variability  or  the  rate  of  decrease  in  heterozygosis  is  approxi- 
mately equal  to  1/2N.  Here,  N  is  the  effective  size  of  the  breeding  population 


GENETIC   DRIFT  •  265 

rather  than  the  total  number  of  individuals  in  the  population,  for  many  will  not 
survive  to  maturity  and  among  those  that  do,  not  all  will  leave  offspring.  Thus, 
the  actual  progenitors  contributing  genetically  to  the  next  generation  may  be  con- 
siderably fewer  in  number  than  the  total  number  of  individuals  living  in  the 
population  at  any  one  time.  Furthermore,  the  breeding  population  may  be  larger 
than  the  so-called  effective  size  of  the  population.  The  breeding  population  will 
equal  the  effective  population  when  equal  numbers  of  males  and  females  are 
mating  at  random  and  contributing  equally  to  the  next  generation.  However,  if 
the  numbers  of  males  and  females  are  unequal,  the  effective  size  will  depend  to 
a  large  extent  on  the  sex  which  is  fewer  in  number.  Thus,  for  example,  in  a 
flock  of  chickens  with  a  few  roosters  serving  a  large  number  of  hens,  the  effec- 
tive size  of  this  population  will  approximate  four  times  the  number  of  roosters 
rather  than  the  total  number  of  breeding  individuals.  Similarly,  in  a  population 
undergoing  periodic  expansion  and  contraction  in  numbers,  the  effective  N  will 
be  much  closer  to  the  minimum  number  than  to  the  maximum.  As  a  simple  ex- 
ample of  the  effect  of  drift,  if  N  were  20,  1/2N  or  1  out  of  40  heterozygous  loci 
on  the  average  would  be  expected  to  become  homozygous  in  the  next  generation. 
It  can  be  seen  that,  continued  over  a  number  of  generations,  genetic  drift  would 
not  only  cause  fluctuations  in  gene  frequency  but  also  would  increase  the  amount 
of  homozygosity  in  the  population. 

Cases  to  illustrate  the  effects  of  genetic  drift  can  be  drawn  from  man. 
American  Indian  tribes  are  known  to  have  formed  rather  small,  isolated,  mating 
populations  in  recent  times  and  are  thought  to  have  formed  such  units  ever  since 
they  first  migrated  to  America.  Human  populations  in  other  parts  of  the  world 
do  not  ordinarily  consist  of  such  small  mating  isolates.  It  is  significant  therefore 
that  whereas  the_.iiequency  of  the  gene  producing  the  A  substance  of  the  ABO 
blood  group  system  ranges  in  the  rest  of  the  world  from  about  15  percent  to 
45  percent,  in_  American  Indian  tribes  it  ranges  from  as  low  as  1  or  2  percent  in 
some  tribes  to  as  high  as  80  percent  in  the  Bloods  and  the  Blackfeet.  A  study  of 
a  genetic  isolate  based  on  religion  has  also  produced  some  interesting  data.  The 
Old  German  Baptist  Brethren,  or  Dunkers,  form  a  community  of  about  300  per- 
sons in  Franklin  County,  Pennsylvania,  but  the  effective  size  of  this  population 
has  been  estimated  to  be  only  about  90.  This  group  was  compared  for  a  number 
of  traits  both  with  the  population  of  the  German  Rhineland,  their  place  of  ori- 
gin, and  also  with  the  population  in  the  United  States  among  whom  they  live 
and  from  whom  they  have  drawn  a  small  fraction  of  their  genes  by  intermarriage. 
The  analysis  showed  quite  clearly  that  this  community  had  'come  to  differ  signifi- 
cantly from  the  populations  both  in  Germany  and  the  United  States  in  several 
but  not  all  of  the  traits  studied — exactly  the  result  that  might  be  expected  with 
genetic  drift.  The  evidence,  therefore,  is  highly  suggestive  that  genetic  drift  does 
play  a  considerable  role  in  determining  gene  frequencies  in  small  isolated  human 
populations. 


266  •  THE    MECHANISM    OF   EVOLUTION 

Genetic  Drift  and  Evolution 

Considerable  discussion  has  arisen  over  the  evolutionary  significance  of 
genetic  drift.  The  debate  has  hinged,  not  so  much  on  whether  genetic  drift  can 
occur,  but  rather  on  whether,  even  if  it  does  occur,  it  has  any  long-range  impor- 
tance in  evolution.  Given  the  facts  of  Mendelian  inheritance,  there  seems  little 
reason  to  doubt  that  random  genetic  drift  can  take  place,  and  if  this  is  so,  it  then 
seems  highly  probable  that  in  particular  instances  or  under  certain  circumstances 
it  has  played  a  role  in  evolution.  The  fate  of  most  small  breeding  populations  is 
undoubtedly  extinction,  due  either  to  the  vicissitudes  that  affect  any  natural 
population,  or  to  the  populations'  inability  to  adapt  to  changing  conditions  be- 
cause of  their  low  variability,  or  simply  to  loss  of  identity  by  interbreeding  with 
members  of  other,  larger  populations.  The  question  still  remains  as  to  the  evolu- 
tionary role  of  the  occasional  small,  divergent  population  that  survives.  The 
available  data,  at  best  not  too  abundant,  have  frequently  been  analyzed  from 
only  one  point  of  view.  For  example,  the  "drifters"  have  sometimes  assumed  that 
apparently  random  gene  frequency  differences  between  different  breeding  popu- 
lations of  the  same  species  are  de  facto  evidence  for  genetic  drift,  and  have  made 
no  attempt  to  determine  whether  these  differences  are  in  any  way  adaptive.  On 
the  other  hand,  the  "selectionists"  may  consider  that  by  proving  that  selection  is 
operating  in  a  population  they  have  thereby  excluded  the  possibility  of  genetic 
drift,  or  they  may  fail  to  make  the  essential  distinction  between  effective  size  and 
population  number.  Furthermore,  drift  seems  likely  to  be  of  greater  significance 
in  some  kinds  of  species  than  in  others.  Top  carnivores,  for  instance,  which  are 
relatively  very  few  in  number  and  apt  to  be  widely  scattered,  might  well  be  more 
likely  subjects  to  investigate  for  the  effects  of  drift  than  some  of  the  species 
studied  thus  far. 

In  actual  populations,  natural  selection  undoubtedly  functions  at  all 
population  sizes,  small  as  well  as  large.  Therefore,  it  may  be  expected  that 
genetic  drift  in  the  absence  of  selection  will  rarely  be  found.  When  selection  as 
well  as  genetic  drift  is  operative,  both  will  tend  to  cooperate,  and  the  deleterious 
genes  in  small  populations  will  be  eliminated  more  rapidly  than  in  large  popula- 
tions in  which  selection  alone  is  effective.  The  reason  is  that  the  less  frequent 
allele  in  a  population  has  a  somewhat  greater  probability  of  decreasing  than  of 
increasing  in  frequency  under  genetic  drift.  Since  the  constant  pressure  of  selec- 
tion will  keep  the  deleterious  gene  at  a  low  frequency,  the  net  effect  of  selection 
plus  drift  is  to  increase  the  rate  of  elimination  of  deleterious  genes.  Natural 
selection  is  the  controlling  factor  in  the  evolution  of  large  populationsr  .which 
usually  remain  quite  heterozygous  and  hence  retain  considerable  variability, 
either  actual  or  potential.  In  small  populations,  the  combined  effect  of  natural 
selection,  genetic  drift,  and  the  greater  likelihood  of  inbreeding  is  to  raise  the 
level  of  homozygosity  and  thus  lower  the  amount  of  variability  in  the  population. 
For  this  reason,  small  populations  may  lose  their  ability  to  adapt  to  changing 


GENETIC   DRIFT  •  267 

conditions  and  become  extinct.  However,  numerous  small  populations  may  also 
come  to  diverge  from  each  other  both  as  a  result  of  different  selection  pressures 
and  the  chance  events  stemming  from  mutation,  genetic  drift,  and  inbreeding. 
Hence  each  population  may  be  regarded  as  a  separate  evolutionary  experiment, 
and  even  though  the  fate  of  most  of  them  is  extinction,  the  possibility  for  rather 
rapid  evolution  in  novel  directions  under  these  circumstances  cannot  be  ignored. 


^SUMMARY 


Changes  in  gene  frequencies  may  occur  in  small  popula- 
tions as  the  result  of  random  genetic  drift.  In  essence,  genetic 
drift  is  a  consequence  of  drawing  a  small  random  sample  of 
gametes  to  form  the  next  generation.  This  sample,  which  by 
chance  may  differ  in  gene  frequency  from  the  gene  frequencies 
in  the  parents,  then  becomes  the  new  gene  pool  from  which  the 
gametes  for  the  next  generation  are  drawn.  In  this  way,  numer- 
ous unpredictable  changes  in  gene  frequency  within  a  population 
may  take  place.  Although  considerable  discussion  of  the  evolu- 
tionary significance  of  genetic  drift  has  been  generated,  there  has 
been  little  doubt  that  drift  can  occur,  and  thus  it  remains  a  factor 
to  be  reckoned  with  in  all  evolutionary  studies. 

SUGGESTED  READING 

Glass,  B.,  1954.  "Genetic  changes  in  human  populations,  especially  those  due  to 
gene  flow  and  genetic  drift,"  Adv.  in  Genetics,  6:95-139. 

Li,  C.  C,  1955.  Population  genetics.  Chicago:  University  of  Chicago  Press. 

Wright,  S.,  1951.  "Fisher  and  Ford  on  the  'Sewall  Wright  effect',"  Amer.  Scientist, 
39/452-458. 


CHAPTER 


28 


The  Origin  of  Subspecies 


New  species  can  arise  in  two  distinct  ways,  shown  dia- 
grammatically  below: 


d 

Time     f  I         c 

b 


b        i 

n      T 


In  I,  only  one  species  exists  at  any  one  point  in  time.  Species  a 
evolves  into  b,  b  into  c,  and  so  on;  it  is  a  "transformation  in 
time."  In  II,  a  single  species  gives  rise  to  two  contemporary 
species;  a  splitting  or  "multiplication  in  space"  has  occurred,  a 
process  known  as  speciation,  in  a  restricted  sense  of  the  word. 
Whereas  the  transformation  of  a  single  species  in  time  is  due  to 
the  combined  effects  of  mutation,  natural  selection,  and  genetic 
drift,  speciation  involves  an  added  problem:  the  origin,  from  a 
single  species,  of  two  or  more  species  that  no  longer  interbreed. 
Once  established,  they  maintain  their  separate  identities  and  pur- 
sue independent  evolutionary  paths.  Our  problem  now  is  to  con- 
sider the  ways  in  which  different  populations  of  the  same  species 
with  essentially  the  same  genetic  composition  can  diverge  from 
each  other.  To  do  so,  it  is  necessary  to  discuss. population  struc- 
ture— that  is,  the  way  in  which  the  individual  members  of  a 
species  are  subdivided  into  breeding  groups. 

Population  Structure 

Some  species  may  be  common  and  widely  distributed, 

268 


THE    ORIGIN    OF    SUBSPECIES  •  269 

forming  one  large,  nearly  continuous  population  over  thousands  of  square  miles 
of  a  continental  land  mass.  The  American  robin  (Turdus  migratorius)  and  the 
red-winged  blackbird  (Agelaius  phoeniceus)  are  species  of  this  type.  However, 
even  though  essentially  continuous  in  their  distribution,  in  that  there  are  no 
gross  barriers  separating  one  segment  of  the  species  from  the  rest,  nevertheless 
mating  is  not  random  over  the  entire  species  range,  for  obviously  one  male  is  not 
equally  likely  to  mate  with  all  of  the  females  in  the  species.  The  chances  that  a 
male  in  Massachusetts  will  mate  with  Michigan  or  Minnesota  females  are  virtu- 
ally nil;  they  are  isolated  by  distance. 

Other  "species  populations  clearly  have  a  discontinuous  distribution. 
A  species  inhabiting  a  series  of  islands  is  perhaps  the  most  clear-cut  example  of 
this  type,  but  a  comparable  situation  is  found  in  species  living  in  a  series  of  iso- 
lated lakes  or  marshes,  in  clumps  of  trees  surrounded  by  prairie,  on  a  particular 
type  of  soil,  or  only  above  a  certain  elevation  in  a  mountain  range.  In  each  case 
each  population  is  quite  clearly  delimited  from  the  other  populations  of  the 
same  species  by  a  zone  in  which  no  members  of  that  species  live. 

A  variety  of  other  population  structures  can  be  visualized,  but  we  shall 
mention  just  one  more,  the  linear  distribution  such  as  might  be  found  in  a 
species  living  in  or  along  a  river.  A  similar  structure  is  found  in  species  living 
along  the  seashore  or  at  a  limited  elevation  along  a  long  mountain  ridge.  Here, 
the  distribution  is  continuous,  but  again  isolation  by  distance  may  be  a  modifying 
aspect. 

The  distribution  pattern  of  a  species  is  determined  by  a  number  of 
factors,  any  one  of  which  may  act  as  ^baxriex-Mlurther  expansion  of  the  species' 
range.  The  barrier  may  be  some  obvious  physical  feature  such  as  an  ocean,  a 
desert,  or  a  mountain  range.  However,  since  an  impassable  barrier  for  one  species 
may  serve  as  a  broad  highway  for  another,  even  barriers  that  seem  obvious  cannot 
be  so  termed  without  reference  to  the  kinds  of  organisms  unable  to  surmount 
them.  Consider,  for  example,  the  different  role  the  ocean  has  played  in  the  dis- 
tribution of  whales  and  elephants.  Climate,  especially  as  related  to  temperature 
and  moisture,  may  set  limits  on  the  range  of  a  species,  and  such  limits  are  quite 
as  rigorous  in  their  way  as  are  the  physical  barriers.  Furthermore,  some  plants  are 
restricted  by  their  soil,  or  edaphic,  requirements  to  only  limited  portions  of  an 
otherwise  suitable  habitat. 

The  ecological  conditions,  which  are  of  course  in  part  determined  by 
the  physical  conditions,  may  also  influence  the  distribution  pattern  of  a  species 
and^serve  as  a  barrier  to  its  expansion.  One  has  but  to  think  of  species  typical 
only  of  the  prairie,  or  of  coniferous  forest,  or  of  deciduous  forest  to  realize  that 
distribution  also  depends  on  the  type  of  habitat  available.  Destruction  of  its 
habitat  means  the  elimination  of  a  species  from  that  area.  For  this  reason, 
present  game  and  fish  management  practices  are  placing  increasing  emphasis  on 
habitat  improvement.  These  habitat  needs  may  be  both  general  and  also  quite 


270  •  THE    MECHANISM    OF    EVOLUTION 

specific.  The  distribution,  for  example,  of  oak-gall  wasps  of  the  genus  Cynips 
was  shown  by  Kinsey  (who  later  became  better  known  for  other  research)  to  be 
dependent  on  the  distribution  of  the  oak  trees  in  which  they  laid  their  eggs.  The 
yellow-headed  and  red-winged  blackbirds  are  closely  related  species,  both  of 
which  breed  in  Minnesota  in  cattail  marshes.  While  the  red-wing  is  found  in 
almost  every  cattail  marsh  available,  the  yellow-head  seems  to  breed  only  in  those 
marshes  where  no  willows  or  other  shrubs  or  bushes  encroach  on  the  edges  of 
the  marsh.  It  is  not  surprising,  therefore,  that  it  is  known  as  a  bird  of  the 
prairies. 

For  genetic  divergence  to  take  place  within  a  species,  it  is  essential  that 
the  original  species  population  be  divided  into  populations  that  are  physically 
isolated  from  each  other.  Jf  they  are  not  isolated,  interbreeding  will  occur  and 
no  divergence  will  be  possible,  for  the  species  will  be  sharing  a  common  gene 
pool,  and  continual  hybridization  will  swamp  any  differences  that  might  arise. 
The  actual  distances  may  be  very  great  or  quite  small,  depending  on  the  species. 
A  few  hundred  yards  of  unsuitable  habitat  may  be  quite  sufficient  to  separate  two 
snail  populations,  while  several  hundred  miles'  separation  may  be  necessary  to 
achieve  the  same  degree  of  isolation  in  birds.  The  essential  factor  is  not  the 
absolute  distance,  but  the  lack  of  opportunity  for  mating  between  members  of 
the  different  populations  because  of  their  separation  in  space.  Some  biologists 
have  argued  that  ecological  divergence  could  occur  without  physical  isolation. 
However,  the  initial  and  crucial  steps  leading  to  divergence  in  ecological  require- 
ments would  be  the  most  difficult  and  would  be  likely  to  occur  only  under  the 
most  favorable  circumstances,  if  at  all. 

At  this  point  it  may  be  worthwhile  to  review  some  of  the  terms  used  to 
describe  the  variability  of  natural  populations.  A  breeding  population  or  Men- 
delian  population  is  a  group  of  individuals  tied  together  by  bonds  of  mating  and 
parentage  and  thus  sharing  a  common  gene  pool.  Since  these  individuals  are  not 
of  a  uniform  genotype  but  are  typically  variable,  the  population  is  polymorphic. 
A  species  is  polytypic  if  composed  of  genetically  distinct  breeding  populations. 
Individuals  living  close  enough  to  one  another  so  that  interbreeding  between 
them  is  possible  are  said  to  be  sympatric  (that  is,  living  in  the  same  country). 
Those  living  at  greater  distances  are  allopatric.  Thus  polymorphic  variability 
should  be  found  in  sympatric. individuals;  if  the  variations  are  found  only  in 
allopatric  populations,  they  are  polytypic. 

Races  or  subspecies  are  biological  units  below  the  species  level.  They 
are  geographically  defined  aggregates  of  breeding  populations  that  differ  from 
one  another  in  the  frequencies  of  one  or  more  genetically  determined  traits.  The 
definition  of  race  or  subspecies  is  rather  fuzzy  because  the  concept  of  race  is 
itself  rather  fuzzy.  For  example,  it  is  impossible  to  say,  without  being  arbitrary, 
just  how  different  two  populations  must  be  to  warrant  subspecific  rank.  Further- 
more, in  some  cases  the  traits  of  a  species  seem  to  change  rather  gradually  across 


THE    ORIGIN    OF   SUBSPECIES  •  271 

the  range  of  the  species  and  a  dine  is  said  to  exist.  These  gradual,  continuous 
changes  are  the  result  of  adaptation  to  similar  gradual  changes  in  such  things  as 
annual  temperatures  or  rainfall.  The  difficulty  in  denning  a  race  increases  in 
species  where  clines  are  found,  for  even  though  the  terminal  populations  may  be 
quite  different,  if  no  sharp  discontinuity  exists,  it  is  extremely  difficult  to  delimit 
racial  boundaries.  Therefore,  the  concept  is  of  limited  usefulness  and  should  be 
applied  with  caution.  To  dignify  all  infraspecific  variation  with  subspecifk 
taxonomic  names  may  serve  only  to  compound  confusion  rather  than  to  clarify  it. 
In  certain  circumstances  the  labels  may  be  of  sufficient  usefulness  to  justify  using 
them,  but  the  underlying  biological  situation  should  be  kept  clearly  in  mind. 

Isolation  and  Subspeciation 

The  brief  discussion  of  population  structure  above  should  serve  to  indi- 
cate that  a  species  population  usually  has  a  discontinuous  distribution.  If  its 
range  is  very  large,  even  a  more  or  less  continuously  distributed  species  does  not 
form  one  large  randomly  mating  population,  simply  because  of  the  distances  in- 
volved. Therefore,  as  a  general  rule,  a  species  is  composed  of  a  number  of  allo- 
patricbreeding  populations,  each  physically  separated  to  some  extent  from  the 
others  and  pursuing  its  own  independent  evolutionary  path.  Even  though  the 
genetic  composition  of  these  populations  may  initially  be  very  similar,  no  two 
environments  are  likely  to  be  biologically  or  physically  identical,  and  thus  the 
selection  pressures  on  these  populations  will  almost  inevitably  be  somewhat  dif- 
ferent. Selection  plus  the  random  aspects  of  mutation  and,  in  small  populations, 
of  inbreeding  and  genetic  drift  will  bring  about  divergence  in  the  hereditary 
characteristics  of  the  formerly  similar  populations.  For  this  reason,  it  is  to  be 
expected  that  most  widely  distributed  species  will  show  variation  among  the  dif- 
ferent breeding  populations  in  different  parts  of  the  range.  These  differences 
may  take  the  form  of  clines,  or,  when  the  variation  is  sufficiently  well  defined, 
different  geographic  races  or  subspecies  may  be  recognized. 

A  somewhat  different  mode  of  origin  for  genetic  diversity  between 
populations,  suggested  by  Mayr,  is  known  as  the  "founder  principle."  Although 
it  does  not  involve  any  new  concepts,  the  known  principles  are  thought  to 
operate  in  a  somewhat  different  way  from  the  usual  method  outlined  above.  In 
brief,  the  suggestion  is ...that l if,  for example,  a  small  population  colonizes  a  pre- 
viously uninhabited  island,  the  gene  pool  introduced  into  the  island  may  differ 
somewhat  from  that  of  the  species  as  a  whole.  As  a  result,  the  selective  value  of 
the^enes  may  be  somewhat  different  from  their  value  in  the  parental  population, 
hecause  of  their  new  genetic  environment  as  well  as  the  new  external  environ- 
ment. Thus,  drift  and  selection  pressures  are  thought  to  account  for  the  some- 
times striking  differences  between  different  island  populations  and  between 
island  populations  and  their  continental  ancestors. 


272  •  THE    MECHANISM    OF   EVOLUTION 

In  order  to  gain  better  insight  into  the  nature  of  the  differences  between 
geographically  isolated  populations,  let  us  consider  a  few  selected  cases  that  have 
been  studied  rather  carefully.  The  coast  tarweed,  Hemizonia  angustifolia,  is  a 
member  of  the  sunflower  family  and  is  found  in  a  narrow  belt  along  the  sea 
coast  of  California.  Of  the  two  races,  one  extends  275  miles  along  the  coast  from 
northern  California  to  south  of  Monterey  Bay;  the  other,  after  a  gap  of  40  miles 
of  unsuitable  habitat  due  to  the  Santa  Lucia  Mountains,  ranges  another  40  miles 
southward.  Although  the  two  races  are  geographically  isolated  from  each  other, 
they  occupy  ecologically  similar  habitats.  Nevertheless,  because  there  are  small 
but  consistent  and  significant  morphological  differences  between  them,  they  have, 
sometimes  been  called  distinct  species.  Plants  of  the  northern  race  have  a  low, 
broad  habit,  slender  open  branching,  and  rather  small  flower  heads.  The  plants 
from  the  southern  race  have  more  erect,  robust  branching,  and  larger  flower 
heads.  The  two  races  cross  easily  and  produce  fertile  ¥t  hybrids.  The  F2  showed 
that  the  slight  differences  between  the  races  were  due  to  numerous  multiple 
factors.  Of  1152  F2  plants  reared,  no  two  were  alike  and  no  plant  was  exactly 
like  either  of  the  parents.  Almost  all  possible  recombinations  of  the  parental 
traits  were  found.  Whereas  57  percent  of  the  F2  individuals  were  as  large  as  the 
parents,  43  percent  were  smaller  in  size,  some  being  as  much  as  1000  times 
smaller  than  other  F2  plants  (Fig.  28-1).  Thus  the  genes  in  these  two  races  have 
diverged  sufficiently  so  that  in  some  combinations  they  do  not  support  develop- 
ment to  normal  size  even  though  the  combinations  are  viable.  However,  fertility 
and  viability  in  the  hybrids  are  sufficiently  good  to  warrant  calling  these  two 
groups  geographical  subspecies  rather  than  separate  species.  Since  both  occupy 
the  coastal  plain,  Clausen,  Keck,  and  Hiesey,  who  made  this  study,  consider 
them  to  form  a  single  ecotype  but  two  geographic  races.  To  what  extent  the 
differences  between  them  may  be  adaptive  and  to  what  extent  they  are  of  chance 
origin  has  not  been  determined. 

Genetic  Differences  between  Subspecies 

A  quite  different  situation  has  been  described  in  the  climatic  or  alti- 
tudinal  races  of  the  cinquefoil,  Potentilla  glandulosa,  a  member  of  the  rose 
family.  This  species  occurs  in  central  California  from  the  lowlands  near  the 
coast  up  to  heights  of  11,000  feet  in  the  Sierra  Nevada.  At  least  seven  climatic 
races  have  been  identified.  The  extreme  types,  the  lowland  and  the  alpine  races, 
are  strikingly  different  both  morphologically  and  physiologically.  The  lowland 
race  grows  throughout  the  year,  but  the  alpine  race  is  winter  dormant  for  nine 
months.  The  alpine  race  is  dwarf  as  are  many  alpine  plants,  but  it  has  large 
flowers;  the  lowland  plants,  though  large  and  robust,  have  small  flowers.  Trans- 
plantation experiments  showed  that  alpine  plants  remained  winter  dormant  for 
two  or  three  months,  even  in  the  lowland  environment,  and  grew  rather  poorly. 


THE    ORIGIN    OF    SUBSPECIES  •  273 


Classes 


320  105 

Total-   1152   F2   plants 


Fig.  28-1.  Genetic  divergence  between  two  geographical  races  of  the  coast  tarweed, 
Hemizonia  angustifolia.  Top,  left,  the  northern  race  (Pi);  right,  the  southern  race 
(P2).  F2,  top,  three  vigorous,  and  bottom,  three  dwarf  segregants.  The  scale  beside 
each  plant  is  10  cm  high.  The  cubes  represent  F2  size  classes,  and  the  numerals 
below,  the  number  of  plants  in  each  class.  The  cube  to  the  left,  50  cm  to  a  side, 
is  comparable  to  the  parents.  The  others  are  35,  25,  15,  10  and  5  cm  respectively. 
(Courtesy  of  Clausen,  Keck,  and  Hiesey.) 


274  •  THE    MECHANISM    OF    EVOLUTION 

The  Coast  Range  plants  failed  to  survive  the  harsh  winter  at  the  alpine  station. 
These  transplantation  experiments  and  others  showed  that  even  though  the 
phenotype  was  modified  to  some  extent  by  the  environment  in  which  the  plant 
was  raised,  the  fundamental  differences  between  these  races  were  genotypic  and 
adaptive  to  the  particular  environment  from  which  the  plants  came.  The  genetic 
basis  for  the  morphological  and  physiological  differences  between  these  races 
was  confirmed  by  the  results  from  crosses  among  them.  Since  the  hybrids  were 
all  vigorous  and  fertile,  no  reproductive  barrier  exists  among  the  various  races. 
In  the  F2  generation,  genetic  recombination  resulted  in  a  complete  reshuffling  of 
the  parental  traits.  Some  of  the  new  F2  combinations  showed  some  rather  surpris- 
ing abilities.  For  example,  some  were  more  vigorous  and  frost  resistant  in  the 
alpine  habitat  than  the  native  alpines.  Many  that  were  well  adapted  to  the  alpine 
climate  had  vegetative  characteristics  of  the  parents  from  the  lower  elevations. 
Some  thrived  at  all  elevations  from  sea  level  to  the  alpine  station,  unlike  any  of 
the  parent  races.  One  recombinant  type  appeared  as  though  it  might  be  well 
adapted  to  the  extreme  maritime  environment,  which  this  species  has  not  yet 
been  able  to  invade  successfully.  The  races  were  distinguished  from  one  another 
by  a  dozen  or  more  easily  recognizable  traits.  Segregation  and  recombination  in 
the  F2  showed  that  these  differences  were  governed  by  multiple  factors  rather 
than  single  gene  differences.  The  results  from  all  of  these  experiments  indicate 
that  the  differences  between  these  races  are  adaptive  and  have  evolved  gradually 
through  the  accumulation  of  numerous  small  genetic  differences.  Furthermore, 
the  potentialities  for  further  evolution  may  be  greatly  enhanced  by  the  release  of 
variability  brought  about  by  hybridization  between  subspecies. 

In  the  leopard  frog,  Rana  pipiens,  a  somewhat  similar  but  in  certain  re- 
spects quite  different  situation  exists.  This  species  ranges  from  northern  Canada 
far  down  into  Central  America.  As  might  be  expected,  individuals  from  different 
geographical  areas  show  morphological  differences,  and  on  these  grounds  a 
number  of  subspecies  have  been  named.  However,  no  general  agreement  about 
the  subspecies  has  been  reached,  for  the  characters  used  are  not  reliable  and  the 
continuous  distribution  of  this  species  makes  lines  of  demarcation  difficult  to  draw. 
Moore  has  shown  that  the  leopard  frog  is  able  to  exist  in  this  wide  range  of 
environments  because  the  southern  populations  of  Rana  pipiens  differ  in  adaptive 
traits  from  the  northern  populations  in  much  the  same  way  that  southern  species 
of  frogs  differ  from  northern  species.  Thus,  for  example,  in  temperature  toler- 
ance and  rate  of  development,  the  northern  frogs  were  able  to  tolerate  and 
develop  normally  at  lower  temperatures  than  southern  frogs,  but  could  not  tol- 
erate the  higher  temperatures  at  which  southern  frogs  still  developed  normally 
(Fig.  28-2).  Data  on  other  traits  gave  comparable  results,  suggesting  that  these 
populations,  too,  have  become  genetically  adapted  to  their  environments.  How- 
ever, unlike  the  crosses  between  races  of  Potentilla  glandulosa,  which  gave 
normal,  fertile  hybrids,  crosses  between  frogs  of  northern  and  southern  origin 


THE    ORIGIN    OF    SUBSPECIES  •  275 


TEMPERATURE    RANGE    FOR 
NORMAL    DEVELOPMENT 


Fig.  28-2.     Geographic  variation  in  embryonic  temperature 

tolerance  in  Rana  pipiens.  Upper  and  lower  limits  are  given 

in  degrees  C.  A  question  mark  indicates  lack  of  data.  (With 

permission  of  Moore.) 

gave  rise  to  inviable  hybrids.  Thus  the  extreme  populations  behave  as  good 
species  toward  each  other.  However,  since  adjacent  populations  are  fully  inter- 
fertile,  no  barrier  to  genetic  exchange  exists  throughout  the  range  of  the  species, 
and  it  is  best  treated  as  a  single  species  in  which  divergent  populations  have 
arisen  owing  to  adaptation  to  local  environmental  conditions,  particularly  with 
respect  to  temperature. 


►  SUMMARY 


A  study  of  the  population  structure  of  a  species  typically 
reveals  that  it  is  composed  of  a  number  of  more  or  less  isolated 
breeding  populations.  Since  the  habitat  is  unlikely  to  be  uniformly 


276  •  THE    MECHANISM    OF    EVOLUTION 

favorable  throughout  the  species'  range,  this  structure  is  to  be 
expected.  The  origin  of  genetically  divergent  groups  or  subspecies 
within  a  species  virtually  requires  some  degree  of  isolation  be- 
tween breeding  populations,  for  otherwise,  any  differences  that 
might  arise  would  be  swamped  by  hybridization.  This  isolation 
should  not  be  thought  of  in  terms  of  any  absolute  distance  be- 
tween populations,  but  rather  as  the  lack  of  opportunity  for 
mating  between  the  members  of  different  groups.  Since  conditions 
are  seldom,  if  ever,  completely  identical,  the  differing  selection 
pressures,  plus  the  random  effects  of  mutation  and  genetic  drift, 
tend  to  bring  about  genetic  divergence  between  the  different 
populations.  The  result  is  the  formation  of  populations  especially 
well  adapted  to  their  conditions  of  existence  and  differing  from 
other  populations  of  the  same  species  living  under  somewhat 
different  environmental  conditions.  Although  the  possibility  of  the 
sympatric  differentiation  of  one  population  into  two  distinct 
breeding  populations  cannot  be  completely  excluded,  it  must,  in 
view  of  the  difficulties  attendant  on  such  an  event,  have  played 
only  a  minor  role  in  the  evolutionary  process.  The  establishment 
of  genetic  differences  between  different  breeding  populations  of 
the  same  species  is  the  first  step  toward  the  origin  of  species. 

SUGGESTED  READING 

Clausen,  J.,  and  W.  M.  Hiesey,  1958.  "Experimental  studies  on  the  nature  of  species. 
IV,  Genetic  structure  of  ecological  races,"  Carnegie  Institute,  Washington, 
D.  C,  Publ.  615. 

Mayr,  E.,  1942.  Systematic s  and  the  origin  of  species.  New  York:  Columbia  Uni- 
versity Press. 

,    1959-    "Isolation   as   an   evolutionary   factor,"   Proc.   Amer.   Philosophical 

Society,  103(2)  :221-230. 

Moore,  J.  A.,  1949.  "Geographic  variation  of  adaptive  characters  in  Rana  pipiens 
Schreber,"  Evolution,  3.T-24. 


CHAPTER 


29 


Hybridization  and  Evolution 


We  have  just  considered  the  role  of  isolation  in  the 
origin  of  subspecies,  and  we  must  now  consider  what  happens  if 
for  some  reason  isolation  breaks  down  and  interbreeding  again 
occurs  between  formerly  isolated  and  divergent  populations.  The 
importance  of  hybridization  to  evolution  has  been  overstressed  by 
some,  who  think  there  is  a  hybrid  under  every  bush  and  often 
that  the  bush  is  a  hybrid,  too.  Others  have  dismissed  it  as  of  no 
significance.  The  truth  probably  lies  somewhere  between  these 
extremes,  with  hybridization  more  important  to  plant  than  to  ani- 
mal evolution.  In  plants  there  is  no  psychological  isolation,  sexual 
reproduction  is  more  efficient  than  in  animals,  and  the  individuals 
are  longer  lived — all  factors  that  contribute  to  successful  hybridi- 
zation. However,  hybrids  in  animals  have  been  identified  in 
natural  populations  of  fresh-water  fishes,  toads,  and  warblers, 
proof  that  hybridization  does  occur  in  animals  as  well  as  in  plants. 

The  breakdown  of  isolation  may  come  about  in  a  variety 
of  ways.  Physical  changes  in  the  environment  due  to  fires,  floods, 
earthquakes,  volcanic  eruptions,  or  other  catastrophes  may  drasti- 
cally alter  the  habitat.  Changes  in  climate,  and  the  resultant 
changes  in  precipitation,  the  retreat  of  glaciers,  land-bridge  for- 
mation, all  may  lead  to  renewed  contact  between  formerly  isolated 
groups.  The  environment  does  not  remain  stable  indefinitely,  but 
undergoes  both  local  and  regional  shifts  in  character  in  many 
ways. 

Changes  in  the  biota  may  also  radically  alter  the  environ- 
ment.  The  goats  introduced  on  Pitcairn  Island  have  kept  the 


277 


278  •  THE    MECHANISM    OF    EVOLUTION 

island  virtually  denuded  of  large  trees.  Of  all  the  species,  however,  man 
has  had  the  greatest  impact  on  the  environment  all  over  the  world.  His 
activities — clearing  forests,  burning  over  land,  planting  crops,  draining  swamps, 
and  building  roads,  railroads,  dams,  homes,  towns,  and  cities — have  dis- 
rupted the  environment  almost  beyond  recognition  or  belief  in  many  in- 
stances. With  him  he  has  carried  weedy  species  of  plants  and  animals  to 
all  parts  of  the  earth.  The  rabbit  with  its  depredations  on  the  range  lands  of 
Australia  is  a  familiar  example.  The  impact  of  such  species  as  man  and  the 
rabbit  is  direct  and  obvious,  but  the  interrelationships  among  organisms  are  so 
complex  and  interwoven  that  a  single  change,  like  a  stone  in  a  pond,  may  set  in 
motion  a  chain  of  events  in  an  ever-widening  circle.  The  classical  example  of  the 
effect  of  the  number  of  spinsters  on  the  red  clover  crop  will  serve  to  illustrate 
this  point  (Fig.  29-1).  Clover  depends  for  fertilization  on  the  bumblebee;  field 
mice  feed  on  bumblebee  nests;  cats  prey  upon  the  mice;  and  it  is  well  known 
that  old  maids  keep  cats  for  company.  Thus,  it  is  obvious  that  the  larger  the 
number  of  spinsters,  the  better  the  clover  crop. 


The  Effects  of  Migration 

The  effects  of  hybridization  will  differ  to  some  extent,  depending  on  the 
degree  of  genetic  divergence  between  the  populations  involved.  Let  us  consider 
first  the  simple  case  in  which  the  populations  differ  very  little.  Imagine  a  popula- 
tion of  mice  on  an  island  a  short  distance  off  the  mainland  coast,  from  which 
migrants  regularly  reach  the  island.  Assume  that  the  frequency  of  the  gene  A  is 
0.4  in  the  mainland  population  but  only  0.2  on  the  island.  The  effect  of  these 
immigrants  on  the  frequency  of  A  in  the  island  population  will  depend  on  their 
genetic  contribution  to  the  island  population,  which  is  measured  by  m,  the  co- 
efficient of  replacement.  The  value  of  m  is  determined  by  the  proportion  of 
gametes  contributed  to  the  next  generation  by  the  immigrants.  The  change,  due 
to  immigration,  in  the  frequency  of  A  on  the  island  is  given  by  the  equation, 

Ap  =  —mQp  —  pnO 
where  p  =  frequency  of  A  on  the  island 

pm  =  frequency  of  A  among  the  immigrants 
m  =  coefficient  of  replacement 

If  m  is  equal  to  10  percent,  then 

Ap  =  -0.1(0.2  -  0.4)  =  -F0.02 
po  +  Ap  =  pi  =  0.20  +  0.02  =  0.22 


HYBRIDIZATION    AND    EVOLUTION  •  279 


&r 


®jtm 


■j?  \r 


Fig.  29-1.     Biological  complexity:   the  effect  of  spinsters  on  the  red  clover  crop. 


When  p  =  pm,  an  equilibrium  will  be  established.  If  the  above  rate  of  immigra- 
tion persists,  it  is  clear  that  an  equilibrium  will  soon  be  reached  and  that  the 
island  population  cannot  retain  its  individuality. 

In  many  respects  the  effects  of  migration  or  gene  flow  are  similar  to 
those  of  mutation,  for  both  mutation  and  migration  introduce  new  genes  into  a 
population.  By  migration,  favorable  genes  or  gene  combinations  can  spread 
throughout  a  species  from  the  population  in  which  they  arose.  Thus,  migration 
tends  to  make  local  populations  more  nearly  alike  in  gene  frequencies  and  to 
prevent  any  significant  local  differentiation  within  a  species.  If  isolation  is  com- 
plete (m  =  0),  each  population  will  pursue  an  independent  course.  For  values 
of  m  other  than  zero,  the  consequences  of  migration  will  depend  on  the  relation- 


280  •  THE    MECHANISM    OF   EVOLUTION 

ship  between  the  amount  of  gene  flow  and  the  factors  such  as  selection  pressure 
and  genetic  drift  that  operate  within  each  breeding  population.  If,  for  example, 
the  intensity  of  selection,  as  measured  in  terms  of  the  selection  coefficient  (j),  is 
greater  than  the  effect  of  immigration  as  expressed  by  the  coefficient  of  replace- 
ment, then  local  gene  frequencies  will  depend  largely  on  selection  pressure,  with 
migration  having  only  a  minor  diluting  effect.  On  the  other  hand,  if  m  is  greater 
than  j,  the  gene  frequencies  in  the  local  populations  will  not  differ  greatly  from 
the  average  frequencies  in  the  total  population. 

Introgressive  Hybridization 

When  hybridization  occurs  between  two  subspecies  or  species,  the  ¥t  is 
usually  quite  uniform  and  intermediate  in  phenotype  to  the  parents.  If  formed, 
the  F2  is  quite  variable,  because  of  the  recombination  of  numerous  gene  pairs. 
However,  the  rare,  naturally  occurring  hybrids  have  a  much  greater  chance  of 
back  crossing  to  one  of  the  parent  species  than  of  mating  with  each  other,  and 
therefore  a  simple  F2  would  only  seldom  be  expected.  Thus,  where  hybridization 
is  taking  place  under  relatively  stable  environmental  conditions,  three  distinct 
groups,  hybrids  and  the  two  parent  species,  are  generally  not  found.  Instead,  the 
parent  species  will  be  somewhat  more  variable  than  in  other  areas  where  they  are 
not  sympatric,  and  each  will  show  some  traits  suggestive  of  the  other  species. 
This  type  of  situation  is  known  as  introgressive  hybridization.  The  nearer  a  back- 
cross  individual  resembles  one  of  the  well-adapted  parents,  the  better  its  chances 
of  survival  in  a  stable  environment,  and  hence  the  more  subtle  the  introgression 
of  the  foreign  genes.  However,  if  the  hybrids  are  formed  in  a  highly  disrupted, 
unstable  environment,  new  and  different  adaptive  types  may  be  formed  that  are 
better  adapted  to  the  new  conditions  than  either  of  the  parents.  Thus  gene  flow 
may  occur,  even  across  partial  interspecific  barriers.  An  example  of  introgression 
has  been  found  in  the  Mississippi  delta  country  of  Louisiana.  Iris  fulva  grew  in 
clay  soil  and  partial  shade  while  Iris  hexagona,  a  closely  related,  but  quite 
different-looking  species,  grew  in  full  sunlight  in  the  tidal  marshes  (see  Fig. 
29-2).  The  clearing  of  the  woodlands  and  the  draining  of  the  swamps  have  led 
to  considerable  introgression  in  these  two  species,  in  some  cases  with  hybrid 
populations  persisting  to  fill  newly  created  ecological  niches.  Numerous  other 
examples  have  been  described  in  plants,  most  of  them  in  areas  disturbed  by  man. 
However,  two  species  of  sugar  maples,  ecologically  distinct  in  southern  Michigan, 
have  been  found  hybridizing  in  a  formerly  glaciated  part  of  Quebec.  When  it  is 
realized  that  many  parts  of  North  America  were  covered  by  glaciers  as  recently 
as  10,000  to  12,000  years  ago  and  that  all  the  animals  and  plants  now  living  in 
these  areas  must  have  reinvaded  them  not  so  very  many  generations  ago,  it  is 
easier  to  visualize  how  rapidly  conditions  may  change  for  a  given  species  and 
how  isolation  may  arise  and  then  break  down. 


HYBRIDIZATION    AND   EVOLUTION  •  281 


Fig.  29-2.  Introgression  in  iris.  Below:  Flowers  and  enlarged 
sepals  of  Iris  fulva  (left)  and  Iris  hexagona  var.  giganti- 
caerulea  (right)  to  the  same  scales.  Above:  Map  of  the  area 
where  these  two  species  are  hybridizing.  H-l  and  H-2  are 
two  somewhat  different  hybrid  colonies.  (With  permission  of 
Anderson. ) 


Polyploidy  and  Evolution 

Introgression  is  possible  only  if  the  hybrid  is  at  least  partially  fertile. 
However,  even  if  hybrid  sterility  blocks  direct  gene  flow,  genes  from  two  dif- 
ferent species  may  still  form  viable,  fertile  polyploids.  Most  natural  polyploids 
are  the  result  of  hybridization  between  two  species,  with  a  subsequent  doubling 


282  •  THE    MECHANISM    OF   EVOLUTION 

in  the  number  of  chromosomes,  and  hence  are  allopolyploids.  Even  many  poly- 
ploids thought  originally  to  be  autopolyploids  derived  from  a  single  species  have 
frequently,  on  closer  study,  been  shown  to  be  allopolyploids. 

In  the  broad  sense  of  the  word  "mutation,"  polyploidy  is  a  mutational 
change.  It  is  the  only  known  method  by  which  cataclysmic  evolution  can  occur, 
giving  rise  to  a  new  species  in  a  single  step,  for  a  new  polyploid  species  is  fertile 
and  true  breeding  yet  is  reproductively  isolated  from  both  parent  species.  How- 
ever, it  is  a  specialized  and  restricted  form  of  evolution,  occurring  primarily  in 
plants  and  involving  the  recombination  of  existing  genes  rather  than  the  creation 
of  anything  truly  new. 

Polyploids  frequently  have  different  distributions  and  different  ecological 
preferences  from  their  diploid  relatives,  and  are  generally  thought  to  be  more 
tolerant  of  extreme  ecological  conditions.  For  example,  in  Biscutella  laevigata  of 
the  mustard  family  Cruciferae,  the  tetraploids  have  a  continuous  distribution  over 
much  of  Europe  including  the  Alps,  the  Carpathians,  and  the  mountains  of 
Italy  and  the  northern  Balkans.  The  diploids  have  a  discontinuous  distribution 
and  are  confined  to  the  valleys  of  the  Rhine,  Elbe,  Oder,  and  upper  Danube.  See 
Fig.  29-3.  The  diploids  are  confined  to  regions  that  were  not  covered  by  the  ice 
sheets  during  the  glacial  period  and  hence  were  open  to  habitation  for  a  long 
time.  The  tetraploids  exist  now  in  the  areas  formerly  covered  by  the  ice  sheet 
and  must  have  invaded  these  areas  from  elsewhere  while  the  diploids  were  ap- 
parently unable  to  do  so.  The  wider  distribution  of  polyploids  may  be  due  to  a 
wider  range  of  adaptability,  which  permits  them  to  invade  and  colonize  areas 
newly  open  to  plants. 

In  several  cases  it  has  been  possible  to  resynthesize  naturally  occurring 
polyploids  and  thus  prove  not  only  their  hybrid  origin  but  also  their  exact 
parentage.  For  instance,  the  mint  Galeopsis  tetrahit  with  In  —  32  has  been  re- 
synthesized  from  G.  pubescens  and  G.  speciosa,  each  with  In  =  16.  The  syn- 
thetic polyploid  is  similar  in  morphology,  cytology,  and  genetics  to  the  natural 
species. 

In  animals,  polyploidy  is  rare  and  must,  therefore,  have  played  only  a 
minor  role  in  animal  evolution.  The  few  known  animal  polyploids  occur  almost 
exclusively  in  hermaphroditic  or  parthenogenetic  species.  Its  rarity  is  very  prob- 
ably due  to  the  separation  of  the  sexes  in  animals,  for  polyploidy  almost  in- 
evitably upsets  the  chromosomal  sex-determining  mechanism.  The  normal  diploid 
female  in  most  animal  species  has  two  sets  of  autosomes  plus  two  X  chromo- 
somes; the  male  has  two  sets  of  autosomes  plus  an  X  and  a  Y  chromosome.  In 
triploid  or  tetraploid  individuals  there  may  be  an  imbalance  between  the  X  and 
the  Y  chromosomes  (XXY,  XYY,  XXXY,  etc.)  or  between  the  sex  chromo- 
somes and  the  autosomes,  so  that  in  most  cases  they  are  intersexes  or  sterile  or 
otherwise  abnormal.  Under  these  circumstances,  maintenance  of  a  stable  poly- 
ploid condition  is  very  improbable.  Since  polyploid  tissues  have  been  observed 


HYBRIDIZATION    AND    EVOLUTION  •  283 


TETRAPLOIDS 
•   all  Forms  known  to  be  letraploid . 
x   forms  not  investigated  but  probably  tetraploid. 
DIPLOIDS 
03    Sep.  gracilis  <t>   Ssp  alsatica  O  Ssp    subaphytla 

A    var  mollis 
FORMS   NOT   INVESTIGATED     BUT     PROBABLY       DIPLOID 


EB    Ssp.  iernen 


A   Ssp.  3ustriaca 


,  ( Ssp  guestphahca 
{Ssp.   teswifolia 


D    rar   paruifoha 


Fig.  29-3.     Detailed  distribution  of  diploid  and  tetraploid  forms 

of  the  cruciferous  plant  Biscutella  laevigata  in  Central  Europe. 

(Adapted  by  Manton  from  Machatschki-Laurich.)  The  thick  black 

lines  represent  the  boundaries  of  the  ice  sheets. 


284  •  THE    MECHANISM    OF   EVOLUTION 

in  diploid  species,  polyploidy  is  at  least  possible  in  animal  cells;  in  fact,  poly- 
ploid animals  have  occasionally  been  reported.  Even  in  man  a  triploid  has  been 
found,  and  Klinefelter's  syndrome,  characterized  by  faulty  development  of  the 
seminiferous  tubules,  has  been  shown  to  be  an  XXY  intersex  condition.  Hence, 
the  abnormal  sexual  development  in  animal  polyploids  appears  to  constitute  a 
major  barrier  to  their  success. 

Since  animal  evolution  has  proceeded  normally  in  many  lines  in  which 
no  polyploids  are  found,  polyploidy  cannot  be  an  essential  part  of  the  evolu- 
tionary mechanism.  On  the  other  hand,  at  least  one  third  of  all  species  of  higher 
plants  are  polyploid,  an  indication  that  polyploidy  has  obviously  played  a  major 
role  in  plant  evolution.  Nevertheless,  it  has  been  suggested  that  major  evolu- 
tionary advances  have  been  confined  to  the  diploid  lines  even  in  plants,  and  that 
polyploids  may  lead  to  evolutionary  dead  ends  because  of  their  greater  pheno- 
typic  stability. 

Evolutionary  changes  involving  major  adaptive  shifts  typically  occur  at 
exceptionally  rapid  rates  under  changing  environmental  conditions.  Mutation 
rates  are  thought  to  be  generally  too  low  to  provide  at  any  one  time  the  vari- 
ability necessary  to  permit  such  rapid  rates  of  evolution.  However,  the  primary 
effect  of  hybridization  between  members  of  different  populations  is  to  increase 
greatly  the  available  genetic  variability  through  genetic  recombination.  There- 
fore, hybridization  has  been  hypothesized  as  being  especially  favorable  to  rapid 
rates  of  evolution.  If  this  is  the  case,  then  hybridization  has  a  peculiarly  signifi- 
cant role  in  the  evolutionary  process.  Furthermore,  the  familiar  phylogenetic 
diagram  in  the  form  of  a  branching  tree  is  incomplete,  for  the  pattern  should  be 
reticulate  as  well  as  branching. 


SUMMARY  < 


Hybridization  between  members  of  different  breeding 
populations  may  result  from  a  breakdown  in  isolation  between  the 
groups.  The  consequences  of  hybridization  depend  upon  a  num- 
ber of  circumstances.  If  the  populations  are  genetically  rather 
similar,  hybridization  may  be  treated  as  migration  or  gene  flow 
from  one  population  to  the  other,  which  will  tend  to  reduce  and 
eventually  eliminate  the  genetic  differences  between  them.  Thus, 
extensive  gene  flow  tends  to  prevent  local  differentiation  of  popu- 
lations within  a  species.  Hybridization  between  species  or  rela- 
tively well-defined  subspecies  may  lead  to  introgressive  hybridiza- 
tion, the  introduction  of  some  genes  from  one  population  into  the 
other.  An  increase  in  genetic  variability  may  thus  occur  without 
a  complete  swamping  of  the  identity  of  the  parental  populations 
by  hybridization.  In  plants,  hybridization  followed  by  chromo- 
some doubling  has  frequently  resulted  in  the  cataclysmic  origin 
of  new  polyploid  species,  reproductively  isolated  from  their 
parents. 


HYBRIDIZATION   AND    EVOLUTION  •  285 

SUGGESTED  READING 

Anderson,  E.,  1949.  Introgressive  hybridization.  New  York:  Wiley. 

Stebbins,  G.  L.,  1950.  Variation  and  evolution  in  plants.  New  York:  Columbia  Uni- 
versity Press. 

,  1959.  "The  role  of  hybridization  in  evolution,"  Proc.  Amer.  Philosophical 

Society,  103(2)  :23 1-251. 


CHAPTER 


30 

Isolating  Mechanisms 


In  the  two  previous  chapters  we  discussed  the  causes  of 
genetic  divergence  between  allopatric  populations  and  the  effects 
of  hybridization  on  such  populations  if  they  again  become  sym- 
patric.  However,  during  periods  of  isolation,  populations  may 
diverge  to  the  point  where  they  do  not  interbreed  even  when 
they  become  sympatric  again.  This  reproductive  isolation  is  due  to 
the  development  of  various  isolating  mechanisms,  which  serve  to 
prevent  or  reduce  the  amount  of  interbreeding.  Geographicaljor 
spatial  isolation  effectively  prevents  gene, exchange  only -so  long 
as  it  exists,  but  isolating  mechanisms  are  under  genetic  control 
and  will  maintain  reproductive  isolation  even  between  populations 
that  again  come  in  contact  with  one^  another.  Virtually  all  of  the 
evidence  suggests  that  the  initial  stages  in  the  development  of 
isolating  mechanisms  must  occur  during  a  period  of  spatial,  isola- 
tion. Therefore,  the  changes  leading  to  reproductive  isolation 
must  be  incidental  to  the  genetic  divergence  that  occurs  during  a 
period  of  isolation.  Crossing  between  members  of  closely  related 
groups  may  be  prevented  in  a  variety  of  different  ways,  of  which 
we  shall  consider  several  for  purposes  of  illustration. 

Types  of  Isolating  Mechanisms 

Ecological  isolating  mechanisms  are  quite  common.  In 
the  deermouse,  Peromyscus  maniculatus,  two  races  inhabit  imme- 
diately contiguous  areas  in  Michigan  but  nevertheless  retain  their 
identities.  One  race  is  confined  to  the  sandy  lakeshore  beaches 

286 


ISOLATING    MECHANISMS  •  287 

wiiile  the  other  inhabits  the  forest  that  starts  just  a  short  distance  back 
from  the  shore.  Their  habitat  preferences  are  evidently  so  well  denned 
that  interbreeding  is  negligible.  The  white  crappie  and  the  black  crappie 
(fresh-water  fish)  inhabit  the  same  streams  in  Indiana,  but  despite  similar 
food  and  other  habits,  they  seldom  interbreed,  for  the  white  crappie  is 
active  by  day  and  the  black  at  night.  Edaphic,  or  soil,  conditions  isolate  the 
spiderwort,  Tradescantia  canaliculata,  which  grows  in  full  sunlight  at  the  tops  of 
cliffs,  from  T.  subaspera,  which  grows  in  the  shade  at  the  bottom.  Given  the 
opportunity,  these  two  species  hybridize  readily. 

Seasonal  isolation  may  be  a  very  effective  barrier  to  gene  exchange.  In 
cockleburs,  for  example,  the  flowering  times  of  two  species  have  become  so  dif- 
ferent that  in  the  same  area  one  species  flowers  only  after  the  other  has  formed 
its  seed  capsules,  and  the  chances  of  crossing  are  nonexistent.  The  American  toad 
(Bufo  americanus)  and  Fowler's  toad  (B.  fowleri)  have  quite  similar  distribu- 
tions and  form  fully  fertile  and  viable  hybrids  in  laboratory  crosses.  However, 
the  two  species  remain  distinct  because  B.  americanus  breeds  early  in  the  season 
whereas.  B.  fowled  breeds  late.  The  occasional  hybrids  between  the  species  are 
Tound  in  situations  where  tKeHhabitat  has  been  disturbed,  indicating  a  difference 
in  ecological  requirements  of  the  species  as  well. 

The  most  complex  behavior  patterns  in  animals  are  generally  in  some 
way  associated  with,-j£production.  In  essence,  courtship  consists  of  a  series  of 
stimuli  and  responses  between  male  and  female,  with  each  response  serving  as  a 
new  stimulus.  It  apparently  functions  primarily  to  arouse  readiness  for  mating 
and  to  synchronize  mating  behavior  rather  than  to  influence  the  choice  of  mates. 
However,  jf  a  male  starts  to  court  a  female  of  an  entirely  different  species,  the 
courtship  is  usually  broken  off  rather  quickly  because  their  behavior  patterns  do 
not  mesh..  In  this  sense,  courtship  does  restrict  the  choice  of  mates.  This  type  of 
isolating  mechanism  is  usually  referred  to  as  sexual  isolation  and  is  based  on 
"psychological"  or  ethological  differences.  The  lack  of  mutual  attraction  has  been 
traced  to  differences  in  scents,  behavior  patterns,  sexual  recognition  signs,  and 
similar  traits.  Ethological  isolation  generally  precedes  the  development  of  sterility 
barriers  and  thus  is  one  of  the  first  isolating  mechanisms  to  appear.  American 
ducks  such  as  the  mallard  and  the  black  duck  cross  readily  in  captivity  and  pro- 
duce fully  fertile  offspring,  but  hybrids  in  nature  are  very  rare.  The  eastern 
meadowlark  {Sturnella  magna)  and  the  western  meadowlark  (S.  neglecta)  are 
much  alike  in  appearance  and  have  broadly  overlapping  ranges  but  nevertheless 
seldom  interbreed  in  the  zone  of  overlap.  In  both  cases  sexual  isolation  must  play 
a  major  role  in  their  reproductive  isolation  even  though  other  factors  undoubt- 
edly contribute  also.  Although  not  a  factor  in  plant  evolution,  the  evolution  of 
behavior  patterns  is  of  great  interest  to  zoologists,  and  comparative  ethology  has 
been  a  rapidly  growing  field  of  study. 

Another  group  of  phenomena  may  be  called  physiological  isolating 


288  •  THE    MECHANISM    OF   EVOLUTION 

mechanisms.  For  example,  the  sperm  of  Drosophila  virilis  males  show  a  lower 
viability  in  the  reproductive  tract  of  alien  females  (D.  americand)  than„irjLiffijT 
own  females.  After  copulation  in  some  species  of  Drosophila,  the  vagina  swells 
greatly  owing  to  the  secretion  of  fluid  into  the  cavity.  This  insemination  reaction 
is  accentuated  to  such  an  extent  in  interspecific  crosses  that  fertilization  and  egg 
laying  may  both  be  blocked  for  days.  In  plants,  the  growth  rate  of  the  pollen 
tube  may  be  slower  than  normal  on  a  foreign  style  or  in  some  cases  the  pollen 
tube  may  even  burst.  Physiological  barriers  of  this  sort  serve  to  limit  or  prevent 
the  union  of  the  gametes  so  that  fertilization  does  not  occur. 

Even  if  fertilization  between  gametes  from  different  populations  takes 
place,  hybrid  inviability  may  intervene  to  prevent  the  development  of  a  viable 
hybrid  organism.  The  zygote  may  cease  development  at  almost  any  stage,  early 
or  late,  or  may  develop  into  a  grossly  deformed  monster.  Such  a  situation  even 
exists  within  a  single  species,  Rana  pipiens,  in  which  hybrids  from  crosses  be- 
tween leopard  frogs  from  northern  and  southern  United  States  are  deformed  and 
inviable.  The  inviability  of  the  hybrids  results  from  a  disharmony  within  the 
embryo,  preventing  normal  development.  In  plants,  another  type  of  disharmony, 
between  the  hybrid  embryo  and  the  seed  coat,  a  maternal  tissue,  sometimes  blocks 
normal  growth.  This  effect  can  be  circumvented  by  removing  the  embryo  from 
the  seed  and  culturing  it  in  vitro.  Embryo  culture  has  been  used  to  rear  several 
plant  hybrids  that  had  never  before  been  successfully  grown. 

Interspecific  crosses  occasionally  result  in  progeny  that  are  all  of  the 
same  sex.  Hybrid  inviability  is  thus  confined  to  just  one  of  the  sexes.  Haldane 
perceived  that  when  one  sex  is  absent  or  rare  or  sterile  in  such  Fx  hybrids,  then 
that  sex  is  the  heterogametic  sex.  This  generalization  is  sometimes  known  as 
Haldane's  rule.  Accordingly,  the  male  hybrids  are  defective  in  most  species 
crosses  except  in  birds,  moths,  and  butterflies,  the  groups  in  which  the  females 
have  ZW  sex  chromosomes  and  hence  are  the  heterogametic  sex. 

In  many  cases,  normal,  vigorous  hybrids  are  formed,  but  are  sterile.  The 
further  exchange  of  genes  is  in  this  way  completely  blocked.  The  mule  is  .the 
classical  example  of  hybrid  sterility.  Any  one  of  a  number  of  conditions  may 
cause  hybrids  to  be  sterile.  In  general,  either  the  sex  organs  fail  to  develop  suffi- 
ciently for  meiosis  to  take  place,  or  else  abnormalities  in  the  meiotic  process  itself 
(for  example,  in  synapsis  or  spindle  formation)  prevent  the  formation  of  normal 
gametes. 

Even  when  vigorous,  fertile  Ft  hybrids  are  produced,  hybrid  breakdown 
in  the  F2  or  back-cross  generations  may  contribute  to  reproductive  isolation.  In 
such  instances  the  subsequent  generations  may  manifest  reduced  vigor  or  fertility 
or  both. 

All  of  the  isolating  mechanisms  mentioned  above  are  in  some  way 
genetically  controlled  and  will  restrict  the  exchange  of  genes  between  different 
groups  of  animals  or  plants.  Once  reproductive  isolation  of  this  sort  is  firmly 


ISOLATING    MECHANISMS  •  289 

established,  the  evolutionary  paths  of  these  groups  will  have  passed  the  point  of 
no  return.  No  longer  will  they  combine  to  form  a  common  breeding  population. 
Generally,  several  isolating  mechanisms  exist  between  different  species;  thus, 
even  though  no  one  mechanism  is  completely  effective,  their  combined  effects 
cause  total  reproductive  isolation.  A  major  problem  is  to  account  for  their  mode 
of  origin,  for  the  achievement  of  reproductive  isolation  is  the  crucial  step  in 
speciation. 

The  Origin  of  Isolating  Mechanisms 

Two  major  theories  have  been  proposed  to  explain  the  origin  of  iso- 
lating mechanisms.  Muller  suggested  that  reproductive  isolation  is  an  incidental 
by-product  of  the  genetic  divergence  that  occurs  during  the  origin  of  subspecies 
and  species  in  allopatric  populations.  In  other  words,  as  the  evolving  populations 
adapt  to  their  different  environments,  a  reshuffling  and  restructuring  of  the 
genes,  the  chromosomes,  and  the  entire  genotype  occurs.  As  a  result,  if  the  popu- 
lations again  become  sympatric,  incompatibilities  causing  reproductive  isolation 
will  already  exist.  Dobzhansky's  theory  is  that  reproductive  isolation  arises  as  a 
result  of  natural  selection.  He,  too,  recognized  the  genotypes  as  integrated  sys- 
tems of  genes  that,  when  drawn  from  different  populations,  may  be  incompatible. 
Hybrids  often  are  poorly  adapted  or  partially  sterile  and  hence  they  will  tend  to 
be  eliminated  by  natural  selection.  Since  selection  eliminates  not  only  the  hy- 
brids but  at  the  same  time  the  genes  of  the  parents  that  hybridized,  selection  is 
acting  against  hybridization  itself.  Those  individuals  that  hybridize  and  those 
genes  favoring  hybridization  will  gradually  be  eliminated  from  the  population. 
Natural  selection  thus  acts  to  reduce  the  wastage  of  gametes  on  the  less-fit  hy- 
brids. These  theories,  of  course,  are  not  mutually  exclusive  but  complementary. 
Although  some  relevant  evidence  is  available,  additional  research  is  needed  to 
evaluate  the  relative  importance  of  these  two  mechanisms  and  to  clarify 
still  further  the  basis  for  the  reproductive  isolation  between  closely  related 
populations. 


Isolating  mechanisms,  which  are  mechanisms  for  main- 
taining reproductive  isolation  between  sympatric  populations,  are 
under  "genetic*  control.  They  may  be  ecological,  seasonal,  etho- 
logical,  or  physiological  barriers  to  fertilization;  or,  if  fertilization 
occurs,  hybrid  invi ability,  hybrid  sterility,  or  hybrid  breakdown  in 
theJF^jimy.. intervene  to  restrict  the  successful  exchange  of  genes 
between  different  populations.  Isolating  mechanisms  have  been 
thought  to  arise  as  an  incidental  by-product  of  the  genetic  diver- 


►  SUMMARY 


290  •  THE    MECHANISM   OF   EVOLUTION 

gence  occurring  during  speciation,  but  it  has  also  been  postulated 
that  natural  selection  against  poorly  adapted  hybrids — in  the  final 
analysis,  selection  against  hybridization  itself — will  tend  to  build 
up  barriers  to  crossing. 

SUGGESTED  READING 

Dobzhansky,  Th.,  1951.  Genetics  and  the  origin  of  species,  3d  ed.  New  York: 
Columbia  University  Press. 

Mayr,  E.,  1942.  Systematic s  and  the  origin  of  species,  New  York:  Columbia  Univer- 
sity Press. 

Stebbins,  G.  L.,  1950.  Variation  and  evolution  in  plants.  New  York:  Columbia  Uni- 
versity Press. 


CHAPTER 


31 


The  Origin  of  Species 


Up  to  this  point  we  have  used  the  word  "species"  with- 
out defining  the  term.  This  vagueness  has  been  purposeful.  Now, 
however,  as  we  begin  our  discussion  of  the  origin  of  species,  a 
definition  is  clearly  in  order.  One  reason  for  having  avoided  the 
question  until  now  is  that  so  many  definitions  exist.  For  example, 
a  serological  species  definition  runs  like  this: 

A  species  of  helminths  may  be  tentatively  defined  as  a 
group  of  organisms,  the  lipid-free  antigen  of  which,  when  diluted 
1:4,000  or  more,  yields  a  positive  precipitin  test  within  one  hour 
with  a  rabbit  antiserum  produced  by  injecting  40  mg  of  dry  weight, 
lipid-free  antigenic  material  and  withdrawn  10  to  12  days  after  the 
last  of  4  intravenous  injections  every  third  day. 

This  definition  certainly  has  precision,  though  just  what 
it  signifies  is  a  little  less  obvious. 

Another  type  of  definition:  "A  species  is  what  a  compe- 
tent taxonomist  considers  to  be  a  species."  The  problem  is  now 
simplified.  Rather  than  classifying  organisms,  we  now  classify 
taxonomists  into  two  categories:  A.  Competent;  B.  Incompetent. 

A  definition  rather  surprising  in  that  it  came  from  a 
geneticist  is,  "Distinct  species  must  be  separable  on  the  basis  of 
ordinary  preserved  material.  This  is  in  order  to  make  it  possible 
for  a  museum  man  to  apply  a  name  to  his  material."  This  state- 
ment is  an  extreme  form  of  a  whole  group  of  definitions  that  use 
morphological  criteria  to  distinguish  between  species.  Further- 
more, it  suggests  that  the  primary  purpose  of  taxonomy  is  to 
facilitate  the  handling  of  museum  specimens. 


291 


292  •  THE    MECHANISM    OF   EVOLUTION 

The  Species  as  a  Biological  Unit 

Another  group  of  definitions  has  come  to  be  known  as  biological  species 
definitions  in  contrast  to  the  morphological  definitions.  Mayr  has  said,  "Species 
are  groups  of  actually  or  potentially  interbreeding  natural  populations  that  are 
reproductively  isolated  from  other  such  groups."  Dobzhansky  wrote,  "Species  are 
formed  when  a  once  actually  or  potentially  interbreeding  array  of  Mendelian 
populations  becomes  segregated  in  two  or  more  reproductively  isolated  arrays," 
or,  more  briefly,  "A  species  is  the  most  inclusive  Mendelian  population."  These 
definitions  treat  the  species  as  a  dynamic  unit,  a  stage  in  the  process  of  evolution, 
and  not  as  a  fixed  static  entity.  The  emphasis  lies  on  the  achievement  of  repro- 
ductive isolation,  with  the  critical  point  in  the  origin  of  species  the  fixation  of 
discontinuity  between  different  populations.  At  the  point  when  genetic  discon- 
tinuity has  been  reached  so  that  two  populations  thenceforward  pursue  inde- 
pendent evolutionary  paths,  species  status  is  attained;  up  to  that  point  they  must 
be  regarded  as  races  or  subspecies.  The  morphological  species  definitions  are 
subjective,  for  they  depend  on  the  judgment  of  the  taxonomist  as  to  the  degree 
of  morphological  similarity  or  difference  worthy  of  species  status.  The  biological 
definitions  are  more  objective,  for  the  behavior  of  the  organisms  themselves  is 
the  factor  that  determines  their  relationship.  The  significant  question  is  whether 
they  do  or  do  not  interbreed.  The  question  is  not  whether  they  can  interbreed 
but  whether  they  actually  do.  Under  experimental  conditions,  many  "good" 
species  can  be  induced  to  cross  and  may  produce  viable,  fertile  offspring;  but  if, 
under  natural  conditions,  little  or  no  gene  flow  occurs  between  them,  their  evolu- 
tionary paths  remain  separate  and  distinct.  Most  North  American  ducks,  for 
example,  are  completely  interfertile,  but  hybrids  are  extremely  rare,  and  so  they 
remain  distinct  species. 

The  primary  objective  of  a  species  definition  is  to  describe  as  well  as 
possible  the  natural  biological  relationships  of  the  populations  involved.  The 
species  is  a  natural  biological  unit  tied  together  by  bonds  of  mating  and  sharing 
a  common  gene  pool.  For  this  reason,  it  has  objective  reality.  All  of  the  other 
taxonomic  categories — subspecies,  genera,  families,  orders,  etc. — are  the  subjec- 
tive creations  of  taxonomists,  for  the  criterion  of  reproductive  isolation  is  in- 
applicable. Among  the  various  taxonomic  categories  the  species  is  unique. 

Although  the  biological  definition  comes  closest  to  describing  the  bio- 
logical realities,  its  application  may  lead  to  difficulties.  For  one,  the  definition  is 
essentially  nondimensional,  not  applicable  to  species  living  in  different  places  or 
at  different  times,  for  it  cannot  be  determined  whether  populations  isolated  from 
each  other  either  in  space  or  in  time  will  actually  interbreed.  To  the  museum 
taxonomist  working  with  dead  specimens,  and  especially  to  the  paleontologist, 
who  has  no  choice  but  to  work  with  nonliving  materials,  this  definition  is  of  no 
value.  However,  again  we  must  return  to  the  objectives  of  a  species  definition. 


THE   ORIGIN    OF   SPECIES  •  293 

Undoubtedly,  if  it  were  possible,  paleontologists  would  prefer  to  use  a  biological 
criterion;  it  would  probably  considerably  simplify  the  nomenclature  in  some 
groups.  And  modern  taxonomy  is  rapidly  moving  beyond  the  point  of  relying 
solely  on  morphological  traits  in  dead  specimens,  but  is  utilizing  information  on 
all  aspects  of  the  biology  of  a  group  in  arriving  at  valid  taxonomic  groupings. 
A  further  implication  of  the  biological  definition  is  that  two  morphologically 
similar  groups  may  be  distinct  species  while  two  groups  widely  divergent  in 
morphology  may  belong  to  the  same  species.  The  reasons  for  this  situation  are 
relatively  simple.  The  morphology  of  an  organism  is  essentially  a  reflection  of 
its  physiology,  and  physiological  changes  leading  to  reproductive  isolation  may 
well  precede  any  major  morphological  changes.  On  the  other  hand,  adaptive 
shifts  leading  to  morphological  changes  may  not  affect  the  basic  reproductive 
pattern  sufficiently  to  lead  to  reproductive  isolation.  These  possibilities  are  not 
merely  theoretical;  certain  reproductively  isolated  species  of  Drosophila  show 
virtually  no  major  morphological  differences.  Drosophila  pseudoobscura  and  D. 
persimilis,  for  instance,  were  formerly  known  as  races  A  and  B  of  D.  pseudo- 
obscura. In  contrast,  European  and  American  sycamores  of  the  genus  Platanus 
are  quite  different  in  appearance  and  have  been  assigned  specific  rank  (P. 
orientalis  and  P.  occidentalism ;  yet  their  interfertility  when  grown  together  indi- 
cates that  species  distinction  may  be  unwarranted.  One  final  difficulty  with  the 
biological  species  definition  is  that  it  is  limited  to  sexually  reproducing  species. 
In  groups  reproducing  asexually,  evolutionary  change  can  occur  only  by  se- 
quential mutations  in  a  given  line,  with  selection  between  lines.  Since  each  line 
of  descent  is  isolated  from  the  others,  each  is  pursuing  an  independent  evolu- 
tionary path,  but  this  hardly  justifies  assigning  specific  rank  to  each.  Sexual 
reproduction  is  practically  universal  among  the  more  complex  or  highly  evolved 
animals  and  plants,  very  probably  because  evolution  can  proceed  more  rapidly  in 
sexually  reproducing  species.  Genes  and  gene  combinations  favored  by  selection 
can  be  combined  and  recombined  in  a  manner  impossible  with  asexual  repro- 
duction, and  hence  adaptation  and  evolution  are  more  flexible  and  more  rapid. 
However,  in  spite  of  the  difficulties  inherent  in  the  biological  species  definition, 
the  morphological  species  and  the  biological  species  generally  agree,  and  the 
exceptional  cases  are  most  instructive. 

Modes  of  Evolution 

The  ways  in  which  species  originate  are  two  or  possibly  three.  Specia- 
tion,  or  the  multiplication  of  species,  leads  to  an  increase  in  the  number  of  con- 
temporary species.  All  the  basic  problems  of  evolution  are  wrapped  up  in  the 
process  of  speciation,  the  way  in  which  one  species  can  split  into  two,  and  to 
this  question  we  have  devoted  most  of  our  attention.  In  brief,  two  or  more 


294  •  THE    MECHANISM    OF   EVOLUTION 

populations  of  a  species,  upon  becoming  physically  isolated,  may  diverge  as  the 
result  of  different  mutation  pressures,  selection  pressures,  random  genetic  drift, 
or  the  net  effect  of  all  three.  If  gene  flow  is  still  possible  through  hybridization, 
migration  pressures  will  be  exerted,  with  the  more  favorable  genes  or  gene  com- 
binations being  disseminated  throughout  the  species.  In  this  fashion  a  complex 
evolutionary  pattern  may  develop,  involving  interpopulation  selection.  However, 
if  the  isolated  populations  diverge  to  the  point  of  reproductive  isolation,  they 
will  have  achieved  the  status  of  distinct  species.  The  process  of  speciation  is 
diagramed  in  Fig.  31-1. 

The  transformation  of  a  species  in  time  is  a  second  mode  of  evolution. 
Simpson  recognizes  two  types  of  transformation,  which  he  has  called  "phyletic" 
evolution  and  "quantum"  evolution.  Phyletic  evolution  involves  a  sustained, 
directional  shift  in  the  average  characters  of  a  population;  it  is,  in  other  words, 


Time 


Adaptive  zone 

Fig.  31-1.     Speciation:    an    increase    in    the   number   of   species, 

achieved  when  the  different  populations  become  reproductively 

isolated.   (After  Simpson.) 


a  line  of  succession  rather  than  an  increase  in  the  total  number  of  existing 
species.  Phyletic  evolution  may  be  due  to  adaptation  to  a  shifting  environment  or 
to  increasing  specialization  or  improved  adaptation  in  a  constant  environment, 
and  may  be  thought  of  as  leading  eventually  to  the  origin  of  new  genera  and 
families.  Diagrammatically,  phyletic  evolution  is  shown  in  Fig.  31-2.  Most 
paleontology  is  devoted  to  the  study  of  phyletic  evolutionary  changes. 

Quantum  evolution,  also  known  as  mega-  and  macroevolution,  is  the 
term  applied  to  the  rapid  shift  of  a  population  to  a  new  equilibrium  distinctly 
unlike  the  ancestral  condition,  thus  leading  to  the  origin  of  higher  taxonomic 
categories  such  as  new  orders  and  classes.  The  origin  of  the  higher  taxonomic 


THE    ORIGIN    OF   SPECIES  •  295 

categories  has  presented  a  problem  because  new  orders  and  classes  generally  ap- 
pear suddenly  in  the  fossil  record,  without  evidence  of  intermediate  fossil  types. 
If  evolution  is  a  gradual  process,  as  both  Darwin  and  modern  theory  hold,  then 
it  might  be  expected  that  fossils  connecting  different  orders  would  be  found  as 
evidence  of  the  gradual  evolutionary  transition  from  one  group  to  another.  Their 
absence  has  led  some  students  of  evolution  to  postulate  that  a  different  mechan- 
ism is  responsible  for  the  origin  of  higher  groups,  and  that  mutation,  selection, 
gene  flow,  and  genetic  drift  are  responsible  only  for  microevolutionary  changes. 
Macroevolution  has,  for  instance,  been  attributed  to  extremely  rare  macro- 
mutations  or  systemic  mutations,  which  have  such  drastic  effects  that  they  give 
rise  to  "hopeful  monsters."  If,  perchance,  a  "monster"  is  adapted  to  a  new  and 
different  way  of  life,  then  the  new  adaptive  type  survives,  and  because  it  is  so 
different,  it  clearly  belongs  in  a  new  taxonomic  group.  For  example,  the  Diptera, 
or  two-winged  flies,  are  clearly  derived  from  the  four-winged  insects,  with  the 


Adaptive  zone 

Fig.  31-2.     Phyletic  evolution:   transformation   in  time  leading  to  origin  of  new 
genera  and  families.   (After  Simpson.) 


gyroscopic  halteres  homologous  to  the  second  pair  of  wings.  Since  a  mutation, 
tetraptera,  is  known  that  converts  a  dipteran  into  a  four-winged  insect,  thus  at 
one  step  excluding  it  from  its  own  order,  it  is  quite  conceivable  that  at  some  time 
in  the  past  the  reverse  occurred  and  the  Diptera  were  derived  from  some  four- 
winged  insect  order  by  a  single  systemic  mutation  giving  rise  at  one  step  to  a 
two-winged  insect  and  hence  to  a  new  order.  However,  such  an  origin  for  higher 
taxonomic  groups  seems  very  improbable.  Aside  from  the  fact  that  no  systemic 
mutations  have  ever  been  demonstrated,  among  the  arguments  against  this  ex- 
planation two  seem  particularly  telling.  It  is  extremely  unlikely  that  a  single 
chance  mutation  would  cause  all  of  the  many  changes  in  the  physiology  and 


296  •  THE    MECHANISM    OF   EVOLUTION 

morphology  of  the  organism  that  would  be  necessary  to  produce  a  type  suffi- 
ciently well  adapted  to  a  new  mode  of  existence  to  be  considered  a  new  order. 
The  differences  between  orders  are  numerous  and  varied  and  have  clearly  in- 
volved the  reorganization  of  the  entire  genotype  rather  than  a  single  mutation, 
no  matter  how  drastic  its  effects.  Furthermore,  if  systemic  mutations  are  so 
precious  and  so  rare,  and  give  rise  to  new  orders  at  one  bound,  then  in  sexually 
reproducing  species  this  lone  individual  of  the  new  order  becomes  a  voice  in  the 
wilderness  seeking  its  mate,  which  does  not  exist,  and  hence  the  order  that 
originated  at  one  step  becomes  extinct  in  one  step.  If  they  are  frequent  enough 
to  occur  contemporaneously,  they  should  have  been  observed  by  now.  On  the 
other  hand,  if  the  mutant  mates  with  members  of  the  parent  species,  it  has  not 
even  achieved  reproductive  isolation  and  can  hardly  be  regarded  as  anything  but 
a  rather  drastic  mutation,  certainly  not  a  new  order. 

The  Origin  of  Higher  Taxonomic  Groups 

If  quantum  evolution  cannot  be  explained  by  systemic  mutation  or  other 
even  less  satisfactory  theories,  how  can  it  be  explained  within  the  existing  theo- 
retical framework  and  why  are  large  gaps  so  common  in  the  fossil  record  be- 
tween the  orders  and  other  higher  taxonomic  categories  ?  In  order  to  discuss  this 
question  it  seems  advisable  to  discuss  preadaptation,  a  word  often  subject  to  mis- 
interpretation. We  shall  use  it,  not  in  the  sense  that  the  organisms  foresee  the 
course  of  their  own  evolution  and  make  the  necessary  adaptive  shifts  before  they 
are  actually  needed,  but  rather  in  the  sense  that  in  the  process  of  becoming 
adapted  to  existing  conditions,  the  organisms  are  modified  in  such  a  way  that 
they  are  also  adapted,  by  chance,  to  some  other  set  of  conditions  under  which 
they  have  never  existed.  The  first  step  in  the  evolution  of  an  internal  parasite, 
for  example,  would  be  the  development  of  the  ability  to  survive  within  the  body 
of  its  host.  Of  necessity,  this  type  of  change  would  have  to  be  preadaptive.  The 
lungfish  are  adapted  to  survive  in  warm,  stagnant  waters  with  a  low  oxygen  con- 
tent because  the  lungs  enable  them  to  obtain  oxygen  from  air.  However,  lungs 
were  preadaptive  for  terrestrial  life.  Hence,  it  appears  that  preadaptation  can 
arise  as  an  incidental  by-product  of  adaptation. 

It  is,  therefore,  entirely  conceivable  that  numerous  preadaptations  may 
exist  at  any  particular  time.  If  a  new  evolutionary  opportunity  or  ecological  niche 
opens  up  to  a  preadapted  population,  it  may  occupy  the  new  niche  relatively 
rapidly  though  still  by  the  gradual  neo-Darwinian  process  involving  mutation, 
natural  selection,  and  possibly  genetic  drift  and  gene  flow.  The  shift  has  been 
visualized  in  terms  of  a  shift  from  one  adaptive  zone  to  another  or  from  one 
adaptive  peak  to  another,  as  shown  in  Fig.  31-3. 

Next  let  us  consider  the  conditions  under  which  evolutionary  changes 


THE    ORIGIN    OF    SPECIES  •  297 

will  occur  most  rapidly.  These  conditions  exist  when  a  species  is  subdivided  into 
many  relatively  small,  partially  isolated  populations.  Each  constitutes  essentially 
a  separate  adaptive  experiment,  for  divergence  is  not  only  possible  but  probable 
as  each  population  adapts  to  its  own  immediate  environment.  Any  particularly 
successful  group  can  spread  rapidly  either  by  migration  and  gene  flow  into 
adjacent  populations  (since  isolation  is  incomplete)  or  by  winning  out  in  inter- 
population  competition.  Striking  new  adaptive  types  appear  most  likely  to  emerge 
when  a  species  range  covers  a  diversified  environment  or  when  the  environment 
itself  is  unstable,  for  then  a  variety  of  selection  pressures  is  exerted. 

Therefore,  the  origin  of  a  higher  taxonomic  group  such  as  an  order  may 
occur  in  a  single,  rather  small,  preadapted  population  of  a  species  to  which  a 
new  ecological  niche  becomes  available.  The  entire  transition  may  occur  in  a 
relatively  short  time,  geologically  speaking,  and  involve  relatively  few  individuals 
compared  to  the  numbers  of  the  old  and  new  orders  that  lived  before,  after,  and 
even  during  the  transition.  Viewed  in  this  light,  it  is  not  at  all  surprising  that  so 


Time 


Adaptive 
zone  1 


Nonadaptive 
zone 


Adaptive 
zone  2 


Fig.  31-3.     Quantum  evolution:   transformation  in  time  leading  to  the  origin  of 
major  higher  categories  such  as  orders.  Note  that  speciation,  phyletic  evolution,  and 
quantum  evolution  may  go  on  simultaneously  and  that  at  all  times  the  basic  evolu- 
tionary unit  is  a  breeding  population.  (After  Simpson.) 


few  transitional  fossil  types  have  been  found.  It  becomes  simply  a  matter  of 
statistics  and  not  a  unique  or  mysterious  process.  It  should  be  noted  that  the 
species  is  the  evolutionary  unit  even  when  it  is  giving  rise  to  higher  taxonomic 
levels.  Evolution  at  all  levels  and  rates  is  due  to  changes  in  gene  frequencies 
within  breeding  populations.  Phyletic  and  quantum  evolution  are  useful  descrip- 
tive terms,  but  they  do  not  imply  a  different  mechanism  of  evolution.  All  three 


298  •  THE    MECHANISM    OF   EVOLUTION 

processes  may  be  concurrent,  and  the  changes  may  be  rapid  or  slow,  requiring 
millions  of  years,  but  the  species  remains  the  basic  unit  of  evolution  under  all 
circumstances. 

The  fossil  record  of  the  horse  family  or  Equidae  is  probably  as  well 
known  as  that  of  any  other  group.  Early  horses  were  small  browsing  animals, 
feeding  on  the  tender  foliage  of  trees  and  shrubs.  They  evolved  and  diversified 
within  the  browsing  adaptive  zone.  One  group  gave  rise  suddenly  to  the  grazing 
horses,  which  fed  on  harsh  grasses,  but  despite  the  wealth  of  fossil  Equidae  mate- 
rial, no  intermediates  are  known.  The  preadaptive  change  in  this  case  appears  to 
be  the  development  of  the  larger  and  higher  crowned  teeth  required  to  grind  up 
the  necessary  amount  of  vegetation  to  support  the  larger  body  that  had  evolved 
in  some  of  the  browsing  horses.  These  teeth  were  preadaptive  for  grazing,  and 
if  the  horse  now  supplemented  its  diet  with  grass,  a  new  ecological  niche  was 
opened  up.  Since  the  prairie  habitat  was  apparently  not  occupied  by  significant 
competitive  herbivores,  selection  pressures  would  then  be  very  strongly  in  favor 
of  the  transition,  for  competition  would  be  keen  for  the  browsing  animals  and 
slight  at  this  point  for  grazing  animals.  The  ultimate  result — adaptation  by  the 
Equidae  to  two  kinds  of  food — permitted  an  increase  in  the  total  number  of 
existing  horses. 

One  point,  however,  should  be  made  in  relation  to  the  models  of 
quantum  evolution.  The  species  must  remain  well  adapted  during  any  and  all 
transitions.  If  it  did  not,  it  would  become  extinct.  For  example,  major  changes 
in  the  form  and  function  of  the  foot  have  not  required  that  the  members  of  a 
species  hobble  around  during  the  transition  period,  which  could  well  have  been 
a  million  years  or  more.  Thus,  the  nonadaptive  zones  or  the  nonadaptive  valleys 
are  misleading.  One  species'  peak  may  be  another  species'  valley;  or,  for  a  given 
species,  the  peak  itself  moves  as  the  species  evolves. 

Although  much  still  remains  to  be  learned,  the  broad  outlines  of  the 
course  of  evolution  and  of  the  mechanism  of  evolution  are  now  fairly  well 
understood.  More  research  on  the  effect  one  species  has  on  its  own  evolution  or 
on  that  of  other  species  is  needed.  Darwin's  theory  of  sexual  selection,  now  more 
or  less  in  limbo,  was  an  attempt  to  study  the  effect  of  a  species  on  its  own  evolu- 
tion (see  Fig.  31-4).  The  cooperative  as  well  as  the  competitive  aspects  of 
natural  selection  are  decidedly  in  need  of  further  study,  for  cooperative  efforts 
may  confer  a  reproductive  advantage  to  a  particular  population  in  competition 
with  other  populations.  Evolution  then  may  reflect  the  effects  of  both  cooperation 
and  competition. 

A  breeding  population  is  an  array  of  genes,  temporarily  embodied  in 
individuals,  but  endlessly  combined  and  recombined  by  the  process  of  sexual 
reproduction.  New  genes  may  be  added  to  the  existing  array  by  mutation  or  by 
gene  flow,  while  random  genetic  drift  may  lead  to  chance  fluctuations  in  the 
existing  gene  pool.  Each  individual,  each  new  combination  of  genes,  is  a  unique 


THE    ORIGIN    OF   SPECIES  •  299 


^Hs 


Fig.  31-4.     Darwin's  finches:  speciation,  following  the  initial  in- 
vasion of  the  Galapagos  Islands  by  finches  from  South  America, 
has  given  rise  to  fourteen  closely  related  but  divergent  species. 
(With  permission  of  Lack.) 

adaptive  experiment  to  be  tested  by  natural  selection.  Similarly,  each  breeding 
population  is  a  unique  adaptive  experiment  to  be  tested  by  natural  selection  in 
competition  with  other  populations.  Although  our  discussion,  by  focusing  pri- 
marily on  events  at  a  single  gene  locus,  has  oversimplified  a  very  complex 
mechanism,  it  has  indicated  the  general  nature  of  the  process  of  evolution. 


300  •  THE    MECHANISM    OF   EVOLUTION 

SUMMARY  < 


Although  many  species  definitions  have  been  proposed, 
most  of  them  can  be  categorized  as  either  "morphological"  or 
"biological."  The  morphological  species  definitions  use  the  degree 
of  morphological  similarity  as  the  criterion  for  distinguishing 
between  species.  The  biological  species  definitions  emphasize 
reproductive  isolation  as  the  essential  criterion  without  regard  to 
morphological  traits.  The  biological  definitions  are  more  objec- 
tive, in  that  the  judgment  is  based  on  the  behavior  of  the  organ- 
isms in  nature  rather  than  on  the  subjective  opinion  of  a  taxono- 
mist.  The  fundamental  question  is  not  whether  the  members  of 
the  two  populations  can  interbreed  but  whether,  in  fact,  they  do. 
If  they  do  not,  they  are  pursuing  independent  evolutionary  paths 
and  must  therefore  be  regarded  as  separate  species.  The  nature  of 
this  definition  makes  it  applicable  primarily  to  sympatric  sexually 
reproducing  organisms.  Three  types  of  evolutionary  change  have 
been  recognized — speciation,  phyletic  evolution,  and  quantum 
evolution — but  all  the  fundamental  questions  about  evolution  are 
related  to  the  process  of  speciation.  Even  the  origin  of  higher 
taxonomic  groups  appears  to  have  been  the  result  of  relatively 
gradual  changes  in  the  hereditary  traits  of  an  interbreeding  group 
of  organisms. 

SUGGESTED  READING 

Amadon,    D.,    1950.    "The    Hawaiian    honey  creepers,"    Bull.    Amer.    Museum    of 

Natural  History,  Vol.  95. 
Clausen,  J.,  1951.  Stages  in  the  evolution  of  plant  species.  Ithaca,  N.  Y. :  Cornell 

University  Press. 
Darwin,  C,  1872.  The  origin  of  species.  New  York:  Mentor  Books  (1958). 
Dobzhansky,   Th.,    1951.   Genetics  and  the  origin  of  species,   3d  ed.  New  York: 

Columbia  University  Press. 
Lack,  D.,  1947.  Darwin's  finches.  New  York:  Cambridge  University  Press. 
Mayr,  E.,  ed.,  1957.  The  species  problem.  AAAS  Symp.  50. 


CHAPTER 


32 


Evolution  of  Genetic  Systems 


Thus  far  we  have  discussed  evolution  almost  exclusively 
in  terms  of  sexually  reproducing,  diploid  species.  This  type  of 
genetic  system  is  undoubtedly  the  most  familiar  reproductive 
mechanism  because  it  is  predominant  among  the  higher  animals 
and  plants.  However,  it  is  by  no  means  the  only  scheme  possible, 
and  many  other  systems  are  known.  In  view  of  these  possibilities, 
the  question  may  well  be  posed  as  to  why  sexuality  and  diploidy 
should  have  come  to  assume  their  predominant  position.  If  evolu- 
tion and  natural  selection  have  affected  the  hereditary  character- 
istics of  organisms  in  such  ways  that  they  become  phenotypically 
better  adapted  to  survive,  and  reproduce  in  their  physical  and 
biological  environments,  there  is  no  reason  to  suppose  that  the 
hereditary  mechanism  itself  is  not  similarly  subject  to  modification 
and  improvement  under  the  influence  of  evolutionary  forces.  The 
fossil  record  gives  some  clues  to  the  course  of  evolution  in 
morphological  traits,  but  no  similar  clues  are  available  for  the 
evolution  of  genetic  systems,  and  conclusions  in  this  area  are 
based  primarily  on  inferences  derived  from  our  knowledge  of 
living  species.  Although  our  surmises  as  to  their  mode  or  sequence 
of  origin  must  be  regarded  as  rather  speculative,  the  fact  that  a 
great  diversity  of  different  genetic  systems  exists  cannot  be 
disputed. 

Genetic  Recombination 

Except  for  several  viruses  in  which  RNA  is  utilized,  the 
control  and  transmission  of  hereditary  traits,  from  viruses  up  to 


301 


302  •  THE    MECHANISM    OF   EVOLUTION 

man,  reside  in  a  single  type  of  compound,  DNA.  In  viruses,  bacteria, 
and  the  blue-green  algae,  the  DNA  does  not  appear  to  be  organized  into 
well-defined  structures,  comparable  in  organization  and  behavior  to  the  chromo- 
somes of  higher  plants  and  animals.  For  a  long  time  it  was  assumed  that  these 
rather  simple,  primitive  organisms  reproduced  only  asexually,  and  that  sexual 
reproduction,  leading  to  genetic  recombination,  had  evolved  from  asexually  re- 
producing species.  However,  the  recent  discovery  of  various  kinds  of  genetic 
recombination  in  bacteria  and  viruses  has  reopened  the  question  of  which  is  the 
more  primitive  condition,  sexuality  or  asexuality. 

The  processes  observed  in  these  simple  organisms  are  in  several  respects 
different  from  sexual  reproduction  in  higher  plants  and  animals.  It  should  be 
noted  and  emphasized  that  sexual  reproduction  has  very  little  to  do  with  sex  in 
the  Freudian  sense.  Though  separate  sexes,  male  and  female,  are  sometimes  in- 
volved, the  essence  of  sexual  reproduction  is  genetic  recombination.  Corn  and 
earthworms,  for  example,  do  not  have  individuals  of  different  sex,  yet  they  re- 
produce sexually.  If  those  processes  resulting  in  genetic  recombination  are  termed 
sexual,  then  the  unusual  forms  of  recombination  in  viruses  and  bacteria  fall 
within  the  realm  of  sexuality. 

Transformation,  the  artificial  recombination  in  Pneumococcus  induced 
when  DNA  from  one  strain  is  added  to  a  culture  of  a  different  strain,  has  al- 
ready been  mentioned  in  an  earlier  chapter.  In  Escherichia  coli,  the  colon  bacillus, 
strains  have  been  found  that  regularly  undergo  genetic  recombination  during 
cellular  contact.  In  this  case,  however,  only  part  of  a  single  "chromosome"  or 
linkage  group  from  one  type  of  strain  (F+  or  Hfr)  enters  an  F~  cell  to  form  a 
partial  heterozygote.  The  size  of  the  transferred  fragment  is  related  to  the  time 
allowed  for  cellular  contact.  Still  another  type  of  genetic  recombination  in  bac- 
teria, known  as  transduction,  is  mediated  by  bacterial  viruses  or  bacteriophages. 
In  transduction,  DNA  from  one  strain  of  bacteria  is  transferred  to  a  different 
strain  by  means  of  the  phage.  Thus  three  rather  different  recombination  mechan- 
isms are  known  in  bacteria:  transformation,  transduction,  and  cellular  fusion. 
They  differ  in  amount  of  DNA  transferred  (least  in  transformation,  greatest 
with  fusion)  and  they  also  differ  from  recombination  in  higher  organisms  in 
that  less  than  a  complete  genetic  complement  may  be  involved. 

A  whole  new  field  of  genetics  has  been  opened  up  by  the  discovery  that 
genetic  recombination  occurs  in  bacteriophages.  Since  a  phage  particle  consists  of 
a  DNA  core  covered  by  a  protein  sheath,  it  is  of  great  interest  that  even  at  this 
simple  level  of  organization  genetic  recombination  is  possible.  Since  the  phages 
multiply  only  in  association  with  a  bacterial  host,  recombination  occurs  only 
when  a  single  bacterium  harbors  more  than  one  type  of  virus  particle.  As  yet 
sexual  processes  have  not  been  reported  in  blue-green  algae  or  in  many  types  of 
bacteria.  However,  it  would  not  be  surprising  if  future  studies  reveal  recombi- 
nation mechanisms  in  additional  groups  of  microorganisms. 


EVOLUTION   OF   GENETIC   SYSTEMS  •  303 

The  evolution  of  the  somewhat  more  complex  unicellular  algae  and 
protozoans  was  accompanied  by  a  more  complex  and  precise  organization  of  the 
genetic  material  itself.  The  genes  were  organized  into  chromosomes  within  a 
nucleus,  and  mitosis  provided  for  the  exact  distribution  of  a  complete  set  of 
hereditary  material  to  each  daughter  cell  following  asexual  cell  division.  Simi- 
larly, meiosis  insured  the  exact  segregation  and  union  of  complete  chromosome 
sets  during  sexual  reproduction.  These  three  advances,  the  origin  of  chromo- 
somes, mitosis,  and  meiosis,  represent  major  steps  in  the  evolution  of  the  genetic 
material. 


Asexual  versus  Sexual  Reproduction 

In  spite  of  the  fact  that  genetic  recombination  is  known  from  even  the 
simplest  and  most  primitive  of  organisms,  it  is  nevertheless  true  that  asexual 
reproduction  is  very  common  among  organisms  at  many  levels  of  organization 
and  complexity.  This  observation  raises  questions  about  the  adaptive  advantages 
and  disadvantages  of  both  asexual  and  sexual  modes  of  reproduction.  On  the 
assumption  that  asexuality  is  the  more  primitive  condition,  then  sexual  repro- 
duction has  arisen  independently  a  number  of  times.  On  the  contrary  assumption, 
that  sexuality  is  more  primitive,  then  asexual  reproduction  has  evolved  repeat- 
edly. In  either  case,  the  indications  are  that  the  genetic  system  has  adaptive  value 
and  has  been  modified  during  the  course  of  evolution.  Arguments  and  theories 
favoring  both  assumptions  have  been  advanced  in  recent  years,  with  perhaps  a 
preponderance  favoring  sexuality  as  the  more  primitive  state  in  view  of  the 
recent  discovery  of  genetic  recombination  in  viruses  and  bacteria.  It  is  appro- 
priate, therefore,  to  consider  now  the  adaptive  significance  of  asexual  reproduc- 
tion. 

Any  asexual  method  of  reproduction  provides  a  means  whereby  rapid 
self-duplication  of  a  particular  genotype  is  possible.  If  this  genotype  is  well 
adapted  to  a  given  stable  environment,  asexual  reproduction  is  then  a  more  effi- 
cient means  of  rapidly  colonizing  this  environment  and  maintaining  a  well- 
adapted  population  there  than  is  sexual  reproduction.  With  genetic  recombina- 
tion a  variety  of  new  genotypes  is  produced,  many  of  which  may  be  poorly 
adapted  to  the  existing  stable  environment.  Asexual  reproduction  will  also  be 
advantageous  where  the  numbers  of  individuals  are  so  small  that  the  probability 
of  encountering  suitable  mating  partners  is  low.  However,  an  asexually  repro- 
ducing population  is  poorly  equipped  to  adapt  to  rapidly  changing  environ- 
mental conditions.  Its  sole  means  of  adapting  to  changed  conditions  is  through 
the  chance  occurrence  of  rare  favorable  mutations.  In  species  such  as  bacteria 
with  large  numbers  and  high  rates  of  multiplication,  this  method  of  adaptation 
may  be  sufficient  as  a  buffer  against  extinction,  but  in  other  species  it  is  not. 

Sexual  reproduction,  on  the  other  hand,  through  the  shuffling  and  sort- 


304  •  THE    MECHANISM    OF   EVOLUTION 

ing  of  genes  into  new  and  different  combinations  with  each  generation,  provides 
a  constant  source  of  new  phenotypes  for  testing  against  the  environment.  Al- 
though at  any  one  time  and  place  there  will  be  a  smaller  proportion  of  well- 
adapted  individuals  than  would  be  produced  by  a  well-adapted  asexual  popula- 
tion, a  sexually  reproducing  population  is  better  able  to  adjust  to  changing 
environmental  conditions  and  to  exploit  new  and  different  ecological  niches.  It  is 
not  surprising,  therefore,  that  among  the  so-called  higher  or  more  complex 
organisms,  sexual  reproduction  seems  to  be  the  mechanism  through  which  this 
complexity  has  evolved. 

If  sexual  recombination  is  truly  the  more  primitive  mode  of  reproduc- 
tion, then  asexual  reproduction  is  a  condition  derived  from  it.  The  asexual  status 
of  many  bacteria,  protozoans,  and  other  groups  of  microorganisms  can  then  be 
interpreted  as  an  adaptive  phenomenon  in  these  organisms.  Many  of  them  exist 
in  relatively  stable  environments  in  which  rapid  asexual  multiplication  is  advan- 
tageous. Others,  living  under  unstable  conditions,  are  nonetheless  capable  of  such 
rapid  multiplication  and  can  adapt  so  readily  via  single  mutations  that  asexual 
reproduction  would  still  have  an  adaptive  advantage  over  any  benefits  from 
genetic  recombination. 

Haploidy  versus  Diploidy 

Although  there  may  still  be  some  doubt  as  to  the  primitive  status  of 
sexual  phenomena,  it  seems  reasonably  clear  that  haploidy  is  the  primitive  state 
from  which  diploidy  has  been  evolved  in  a  number  of  different  unrelated  groups. 
At  the  level  of  organization  above  the  viruses,  bacteria,  and  blue-green  algae— 
namely,  the  flagellates  and  the  green  algae — the  genetic  material  is  organized 
into  chromosomes  that  undergo  mitosis  and  meiosis.  The  most  primitive  flagel- 
lates and  green  algae  are  haploid;  the  only  diploid  cell  is  the  zygote,  and  this 
cell  undergoes  two  meiotic  divisions  that  immediately  restore  the  haploid  condi- 
tion. In  the  evolution  of  both  higher  plants  and  animals,  there  has  been  a  defi- 
nite trend  toward  prolongation  of  the  diploid  phase.  In  other  words,  the  interval 
between  fertilization  and  meiosis  has  increased,  with  a  number  of  mitotic  divi- 
sions of  the  diploid  nucleus  intervening  before  meiosis.  This  observation  raises 
at  once  the  question  of  the  adaptive  advantages  of  diploidy. 

The  Metazoa  and  some  groups  of  Protozoa  are  completely  diploid  ex- 
cept for  the  gametes;  that  is,  meiosis  is  deferred  until  just  prior  to  gamete  forma- 
tion. In  plants,  a  similar  situation  exists  in  the  diatoms,  yeasts,  certain  green 
algae,  and  some  of  the  brown  algae.  Among  the  algae,  the  haploid  life  cycle  has 
frequently  given  rise  to  an  alternation  of  haploid  and  diploid  generations  that 
are  morphologically  very  much  alike.  In  this  case  the  zygote  divides  mitotically 
to  form  the  plant  body,  but  the  deferred  meiosis,  when  it  occurs,  produces 


EVOLUTION    OF   GENETIC    SYSTEMS  •   305 

haploid  spores  rather  than  gametes.  The  spores  then  germinate  and  develop  into 
a  haploid  organism  similar  in  form  to  the  diploid.  It  appears  that  from  this  type 
of  life  cycle,  known  as  an  isomorphic  one,  two  different  types  of  heteromorphic 
life  cycles  have  been  derived.  The  predominant  diploid  type  is  found  in  the 
vascular  plants  and  some  of  the  more  complex  brown  algae.  A  predominantly 
haploid  life  cycle  is  found  in  a  few  algal  groups  and  in  the  mosses  and  liver- 
worts. The  early  theory  that  the  evolution  of  a  predominant  diploid  generation 
made  possible  the  invasion  of  the  land  by  plants  now  appears  to  be  incorrect. 
For  one  thing,  the  complex  marine  brown  algae  also  have  a  predominant  diploid 
phase,  whereas  many  terrestrial  fungi  have  retained  the  haploid  condition. 
Furthermore,  the  bryophytes,  supposedly  representative  of  a  stage  intermediate 
between  the  haploid  algae  and  the  predominantly  diploid  vascular  plants,  are 
apparently  more  recent  in  origin  than  the  oldest  vascular  plants  and  represent  an 
evolutionary  dead  end  rather  than  a  transitional  form.  Therefore,  it  appears  that 
the  adaptive  advantages  of  diploidy  must  be  sought  elsewhere  than  in  its  rela- 
tionship to  the  invasion  of  the  land. 

In  a  haploid  organism,  the  genotype,  whatever  it  may  be,  is  immediately 
expressed.  All  of  the  genotypes  in  a  population  are  exposed  to  selection  at  all 
times,  and  little  variability  can  be  retained  since  all  mutants  unfavorable  at  the 
moment  will  be  eliminated.  A  diploid,  however,  may  carry  a  considerable  amount 
of  unexpressed  variability  in  the  form  of  recessive  genes  in  the  heterozygous 
condition.  A  portion  of  this  variability  will  be  released  and  exposed  to  selection 
each  generation  owing  to  genetic  recombination.  In  this  way  a  population  retains 
its  ability  to  adapt  to  changing  environmental  conditions  while  at  the  same  time 
remaining  well  adapted  to  the  prevailing  conditions.  The  flexibility  should  not 
be  regarded  as  simply  dependent  upon  the  appearance  of  new  homozygous  re- 
cessive mutant  types,  however,  for  diploidy  also  opens  up  the  possibility  for 
interallelic,  epistatic,  and  heterotic  effects  which  may  be  of  considerable  im- 
portance. In  general  diploidy  is  associated  with  the  more  complex  organisms 
that  have  a  long,  precisely  integrated  sequence  of  development.  In  haploids,  evo- 
lution is  primarily  dependent  upon  the  appearance  of  suitable  favorable  muta- 
tions. Diploidy,  through  gene  recombination  and  interaction,  permits  the  forma- 
tion of  new  and  different  integrated  systems  of  genes  without  serious  loss  of 
fitness.  The  effects  of  most  single  gene  mutations  on  a  complex  developmental 
sequence  are  deleterious,  and  in  organisms  with  low  reproductive  rates  and  a 
long  developmental  period,  favorable  individual  mutations  would  customarily  be 
too  rare  to  give  adequate  adaptive  flexibility.  Thus,  diploidy  would  appear  to  be 
an  adaptive  means  of  conserving  and  releasing  variability  in  higher  organisms. 
In  the  mosses  and  liverworts  the  predominant  haploid  gametophyte  may  have 
evolved  in  relation  to  their  pioneering  tendency,  for  a  well-adapted  initial  in- 
vader can  quickly  produce  a  colony  of  similarly  well-adapted  descendants. 


306  •  THE    MECHANISM    OF   EVOLUTION 

The  Separation  of  the  Sexes 

We  have  already  seen  that  genetic  recombination  has  been  observed  in 
even  the  simplest  of  organisms.  The  evolution  of  organisms  of  greater  com- 
plexity has  been  accompanied  by  the  evolution  of  more  complex  systems  for 
ensuring  sexual  reproduction.  In  the  Protozoa,  two  types  of  sexual  process  are 
known.  In  conjugation,  a  temporary  contact  between  two  protozoans — for  ex- 
ample, paramecia — permits  nuclear  exchange.  In  syngamy,  an  actual  fusion  of 
sex  cells  or  gametes  takes  place  to  form  a  zygote.  In  some  cases  the  fusing 
gametes,  known  as  isogametes,  are  identical  in  size  and  form  to,  and  little  dif- 
ferent from,  the  parent  cells.  In  other  species,  the  sex  cells,  called  anisogametes, 
are  similar  in  form  but  quite  different  in  size,  while  in  still  others  differentiation 
of  the  gametes  into  sperm  and  egg  cells  has  occurred.  All  of  these  types  of  repro- 
duction have  been  observed  in  one  flagellate  group,  the  Phytomonadina,  and 
suggest  how  the  differentiation  of  sex  cells  could  have  taken  place. 

In  the  colonial  flagellate,  Volvox,  a.  single  colony  is  capable  of  pro- 
ducing both  sperm  and  egg  cells.  The  production  of  two  kinds  of  gametes,  sperm 
and  egg,  by  a  single  individual  is  known  as  hermaphroditism.  Hermaphrodites 
are  found  throughout  the  plant  kingdom,  though  some  plants  such  as  willows  or 
the  ginkgo  have  separate  sexes.  Hermaphroditism  is  widespread  among  animals 
though  not  so  common  as  in  plants;  in  such  important  groups  as  nematodes, 
insects,  and  vertebrates  it  is  rare  or  absent.  Because  it  is  so  common,  particularly 
among  the  lower  animals  and  plants,  it  appears  that  hermaphroditism,  among 
multicellular  animals  and  plants  at  least,  is  the  ancestral  condition  from  which 
the  separation  of  the  sexes  has  been  derived.  Furthermore,  the  separation  of  the 
sexes  has  even  been  obtained  experimentally  in  hermaphroditic  species — for 
example,  in  corn — through  the  suppression  of  functional  male  flowers  in  one 
type  of  plant  and  functional  female  flowers  in  another.  (Species  with  separate 
sexes  are  frequently  referred  to  in  the  literature  as  bisexual,  an  unfortunate  and 
confusing  choice  of  terms  since  bisexual  is  synonymous  with  hermaphroditic.) 

Sex  Determination 

In  hermaphrodites,  such  as  corn  or  an  earthworm,  male  and  female  sex 
cells  are  produced  by  an  individual  with  a  single  genotype.  In  this  case  sexual 
differentiation  cannot  be  determined  genetically,  but  rather  by  subtle  differences 
in  the  internal  environment  comparable  to  those  leading  to  the  differentiation  of 
other  organs  of  the  body. 

In  species  with  separate  sexes,  a  variety  of  methods  of  sex  determination 
have  evolved.  Here,  too,  environmental  sex  determination  occurs.  The  best- 
known  example  comes  from  the  marine  echiurid  worm,  Bonellia.  If  the  free- 
swimming  larva,  when  it  settles  to  the  sea  bottom  to  undergo  further  develop- 


EVOLUTION    OF   GENETIC   SYSTEMS  •  307 

ment,  happens  to  land  on  the  proboscis  of  a  female,  it  will  enter  the  body  of  the 
female  where  it  differentiates  into  a  minute  male,  living  a  parasitic  existence  in 
the  nephridium  near  the  uterus.  If  the  larva  lands  on  the  sea  bottom,  it  differ- 
entiates into  a  free-living  female  some  500  times  as  large  as  the  male.  The  en- 
vironmental nature  of  sex  determination  in  this  species  can  be  demonstrated  by 


lobes  of 
proboscis 


ciliated  groove 


male  on 
proboscis 


mouth 


body 


anus 


Fig.  32-1.     The  echiurid  marine  worm  Bonellia, 

showing    the    vast    size    difference    between    the 

sexes    despite    environmental    sex    determination. 

(With  permission  of  Begg.) 


rearing  larvae  in  sea  water  containing  female  proboscis  extract.  All  of  the  larvae 
then  become  males.  See  Fig.  32-1. 

In  the  majority  of  species  with  separate  sexes,  sex  determination  has 
been  brought  under  genetic  control.  A  number  of  different  types  of  genetic  sex 
determination  have  been  identified.  The  most  familiar  type  involves  a  hetero- 
gametic  male.  In  this  situation  the  male  carries  two  different  kinds  of  sex  chro- 
mosomes, the  X  and  Y,  and  produces  two  kinds  of  sperm,  bearing  either  an  X 


308  •  THE    MECHANISM    OF   EVOLUTION 

plus  the  autosomes,  or  a  Y  plus  the  autosomes.  The  XX  females  produce  only 
one  type  of  egg,  having  a  single  X  and  a  set  of  autosomes.  A  variation  is  found 
in  some  species  in  which  the  females  are  XX  and  the  males  XO,  having  one  less 
chromosome  than  the  females. 

In  the  heterogametic  female  type  of  sex  determination,  it  is  the  female 
that  has  two  different  kinds  of  sex  chromosomes,  conventionally  called  Z  and  W. 
Consequently,  the  female  produces  two  kinds  of  eggs.  Here,  too,  a  ZO  modifica- 
tion has  been  demonstrated  in  some  species.  Heterogametic  females  are  found  in 
moths  and  butterflies,  in  birds,  and  in  some  fishes.  Heterogametic  males  are 
found  in  most  other  groups  with  separate  sexes. 

The  work  of  Bridges  on  sex  determination  in  Drosophila  melanogaster 
led  to  the  development  of  the  balance  theory  of  sex  determination.  As  a  result 
of  his  findings  he  concluded  that  the  presence  of  two  X  chromosomes  was  not 
alone  sufficient  to  determine  femaleness  nor  were  an  X  and  a  Y  sufficient  for 
maleness.  Rather,  sex  was  influenced  by  the  autosomes  as  well  as  the  sex  chromo- 
somes and  the  significant  feature  was  the  ratio  of  X  chromosomes  to  haploid  sets 
of  autosomes.  The  basis  for  his  conclusion  was  a  study  of  the  sexual  character- 
istics of  flies  with  abnormal  numbers  of  sex  chromosomes  and  autosomes.  Some 
of  the  types  he  obtained  were  as  follows : 


chromosome  complement 

(X  =  X  chromosomes; 

ratio 

A  =  sets  of  autosomes) 

X/A 

Phenotype 

3X  :  2A 

1.5 

superfemale 

3X  :  3A 

1.0 

normal  triploid  female 

2X  :  2A 

1.0 

normal  diploid  female 

2X  :  3A 

0.67 

intersex 

IX  :  2A 

0.50 

normal  male 

IX  :  3A 

0.33 

supermale 

Of  particular  interest  is  the  intersex  shown  above.  It  has  two  X  chromosomes, 
but  is  not  a  normal  female  since  the  balance  between  sex  chromosomes  and  auto- 
somes has  been  upset.  All  of  the  types  observed  were  consistent  with  the  rule 
that  an  X/A  ratio  of  1.0  or  above  resulted  in  a  female  (normal  or  super)  and  a 
ratio  of  0.5  or  below  in  a  male  (normal  or  super).  Ratios  between  0.5  and  1.0 
produced  intersexes,  showing  varying  admixtures  of  male  and  female  traits.  The 
subsidiary  role  of  the  Y  chromosome  in  Drosophila  is  shown  by  the  fact  that  an 
XXY  individual  with  two  sets  of  autosomes  is  a  fertile  female: 

In  the  bryophytes  (the  mosses  and  liverworts)  a  somewhat  different 
type  of  chromosomal  sex  determination  has  been  observed — the  heterozygous 
sporophyte.  In  this  case  the  diploid  sporophyte  is  neither  male  nor  female  but 
carries  an  X  and  a  Y  chromosome  as  well  as  the  autosomes.  The  spores  produced 
by  the  sporophyte  are  of  two  kinds:  X-bearing  spores  develop  into  female 
gametophytes;  Y  bearing,  into  male  gametophytes. 


EVOLUTION    OF   GENETIC   SYSTEMS  •  309 

Even  more  significant  is  the  type  of  sex  determination  exemplified  by 
Melandrium  album,  a  member  of  the  pink  family.  In  this  species,  some  plants 
bear  only  male  flowers  and  others  only  female  flowers.  The  females  have  two  X 
chromosomes  plus  two  sets  of  autosomes;  the  males,  an  X  and  a  Y  chromosome 
in  addition  to  the  autosomes.  However,  sex  determination  in  polyploids  of 
Melandrium  has  shown  the  mechanism  to  be  different  from  that  in  Drosophila. 
In  Melandrium,  the  Y  chromosome  is  male  determining.  As  long  as  the  Y  is 
absent,  any  ratio  of  X  to  A  in  diploids,  triploids,  or  tetraploids  will  produce 
fertile  female  plants  and  no  intersexes.  A  single  Y  is  sufficient  to  produce  male 
plants  even  in  triploids  and  tetraploids.  Thus,  for  example,  the  following  types 
are  all  male  plants : 

diploid  2A— X— Y 

triploid  3A— X— 2Y 

3A— 2X— Y 

tetraploid  4A— 2X— 2Y 

4A— 3X— Y 

None  are  intersexes,  though  occasionally  a  male  plant  will  bear  an  hermaphro- 
ditic flower.  Thus,  quite  a  different  use  is  made  of  the  XY  mechanism  in 
Melandrium  and  in  Drosophila.  In  Melandrium,  the  X  chromosome  seems  to 
bear  genes  for  femaleness,  the  Y  carries  genes  for  maleness,  and  the  autosomes 
are  without  apparent  influence  on  sexuality.  In  Drosophila,  the  factors  for 
femaleness  seem  to  be  borne  on  the  X  chromosomes,  those  for  maleness  on  the 
autosomes,  and  the  Y,  aside  from  an  effect  on  fertility,  seems  to  have  little  influ- 
ence. The  work  on  Melandrium  has  recently  assumed  new  interest  with  the  dis- 
covery that  sex  determination  in  mice  and  men,  and  probably  in  other  mammals, 
is  similar  to  that  in  Melandrium  and  not  like  that  in  Drosophila.  This  conclusion 
is  based  on  the  discovery  that  sterile  human  females  with  a  condition  known  as 
Turner's  syndrome  are  XO  diploids.  A  fruit  fly  of  this  constitution  would  be 
phenotypically  male.  Furthermore,  sterile  human  males  with  Klinefelter' s  syn- 
drome are  XXY  and  diploid  for  the  autosomes.  As  mentioned  above,  in 
Drosophila  such  individuals  are  phenotypic  females  and  not  males.  Thus  in  man 
the  Y  chromosome  is  male  determining. 

In  the  Hymenoptera  (the  ants,  wasps,  and  bees),  still  another  type  of 
sex  determination  exists.  Here  the  female  is  diploid;  the  male,  haploid.  The  sex 
of  an  individual  depends  upon  whether  the  egg  is  fertilized.  Fertilized  eggs 
develop  into  females;  unfertilized  eggs  develop  parthenogenetically  into  haploid 
males.  Hence,  whereas  in  most  groups  the  sex  ratio  is  fixed,  in  the  Hymenoptera 
it  may  vary  considerably.  In  the  social  insects  especially,  a  great  preponderance  of 
females  may  be  produced.  A  haploid  male  receives  a  single  haploid  set  of  chro- 
mosomes from  his  mother  and  passes  it  intact  to  all  of  his  daughters;  he  has  no 


310  •  THE    MECHANISM    OF   EVOLUTION 

father,  and  he  fathers  no  sons  of  his  own.  The  first  meiotic  division  is  abortive; 
the  second  produces  two  identical  functional  sperm.  In  fact,  all  of  his  sperm  cells 
are  genetically  the  same,  for  there  can  be,  of  course,  no  synapsis  or  crossing  over. 

Sexual  Differentiation 

In  the  honey  bee  there  are  two  kinds  of  females,  the  workers  and  the 
queens.  The  workers  ordinarily  do  not  reproduce,  but  the  queen  mates  and  lays 
the  eggs  for  the  entire  colony.  Genetically,  the  queens  and  workers  are  the  same. 
The  differences  in  morphology  and  fertility  between  them  have  been  traced  to 
the  kind  of  food  they  receive  as  larvae.  Larvae  destined  to  become  queens  are 
fed  royal  jelly,  a  food  far  richer  in  pantothenic  acid,  a  vitamin,  than  the  food 
given  to  worker  larvae.  The  honey  bee  provides  an  insight  into  the  relationship 
between  sex  determination  and  sexual  differentiation.  Even  though  both  workers 
and  queens  are  genetically  determined  females,  the  workers  are  sterile  and  only 
the  queens  become  functional  females.  The  sexual  differentiation  of  the  two 
groups  is  modified  by  environmental  factors.  Hence,  although  in  species  with 
chromosomal  sex  determination  the  sex  of  the  individual  is  determined  at  the 
time  of  fertilization,  subsequent  events  may  modify  or  even  inhibit  normal 
sexual  differentiation. 

A  variety  of  influences  may  affect  sexual  differentiation  to  the  extent 
that  sexual  anomalies  result.  The  Drosophila  intersexes  resulting  from  chromo- 
somal imbalance  have  already  been  mentioned.  They  show  a  curious  blending  of 
male  and  female  traits,  the  gonads  and  the  secondary  sexual  characteristics  being 
intermediate  in  form.  Another  quite  different  type  of  intersex  is  the  gynandro- 
morph.  In  these  peculiar  individuals,  one  part  of  the  body  is  male  and  the  other 
is  female.  The  most  striking  cases  have  been  found  in  insects  because  the  insects 
evidently  do  not  have  an  endocrine  system  responsible  for  the  circulation  of  sex 
hormones  throughout  the  body.  A  clear-cut  line  of  demarcation  exists  between 
male  and  female  sectors.  Thus,  each  cell  is  autonomous  with  respect  to  its  sexual 
differentiation.  The  differences  arise  when  developmental  accidents  lead  to  differ- 
ences in  the  sex  chromosome  complement  in  different  body  regions.  In  Dro- 
sophila, occasional  individuals  are  male  on  one  side  and  female  on  the  other 
(Fig.  32-2).  These  individuals  began  as  genetic  females,  but  the  loss  of  an  X 
chromosome  from  one  of  the  nuclei  at  the  two-cell  stage  resulted  in  the  gynan- 
dromorph. 

One  of  the  more  surprising  phenomena  in  sexual  differentiation  is  sex 
reversal.  Frogs  and  toads  are  particularly  subject  to  this  type  of  transformation. 
For  example,  it  was  found  that  a  temperature  of  32°  C  during  development 
would  cause  genetically  female  frogs  to  develop  into  fertile  males.  It  then  be- 
came possible  to  mate  two  genetic  females,  one  a  normal  XX  female,  the  other 
also  XX  but  male.  Since  only  X-bearing  gametes  are  possible,  only  female  off- 


EVOLUTION   OF   GENETIC   SYSTEMS  •  311 


Fig.  32-2.  A  gynandromorph  in  Drosophila,  the  left  half 
female,  the  right  half  male.  On  the  male  side,  note  the  sex 
comb  on  the  right  foreleg,  the  dark  tip  to  the  abdomen  and 
the  mutant  trait,  singed  bristles,  all  of  which  are  absent  from 
the  female  half.  (With  permission  of  Stern. ) 

spring  should  result  under  normal  developmental  conditions;  and  indeed,  among 
a  large  progeny,  no  males  were  found. 

A  rare  situation  in  chickens  offers  an  even  more  spectacular  type  of  sex 
reversal.  In  these  cases  a  normal  hen  gradually  assumed  the  appearance  and  be- 
havior of  a  rooster  and  actually  fathered  chicks.  Only  one  ovary  in  a  normal  hen 
is  functional,  and  when  this  ovary  was  destroyed,  the  primary  sex  cords  in  the 
other  vestigial  gonad  differentiated  into  a  testis.  The  male  sex  hormone  from  the 
testis  then  induced  the  changes  in  the  secondary  sexual  traits  (Fig.  32-3). 

Another  example  of  the  role  of  the  sex  hormones  in  sexual  differentia- 
tion in  vertebrates  comes  from  cattle.  When  twin  calves  of  opposite  sex  are  born, 
the  female  is  almost  always  sterile  and  is  called  a  "freemartin."  The  sex  organs 
are  usually  modified,  and  in  extreme  cases  the  ovaries  have  been  transformed 
into  structures  resembling  testes.  In  twin  cattle,  fusion  (anastomosis)  of  the 
placental  blood  vessels  occurs  and  so  to  some  extent  their  bloods  are  mixed. 
Since  the  hormone  system  causing  male  differentiation  comes  into  play  somewhat 
earlier  than  the  female  system,  the  female  twin  is  affected  by  the  male's  hormones 
before  her  own  hormonal  system  becomes  effective.  The  female  is  transformed 
into  an  hormonal  intersex  but  does  not  become  a  functional  male. 


312  •  THE    MECHANISM    OF    EVOLUTION 


Fig.  32-3.     Sex  reversal.  A  female  fowl  whose  ovary  was  removed  when 
thirteen  days  old  resembles  at  maturity  a  typical  cock.  (Courtesy  of  Snyder 

and  David.) 

Still  another  type  of  intersex  has  been  discovered  in  the  gypsy  moth, 
Lymantria  dispar.  Crosses  between  males  and  females  from  the  same  locality 
produce  normal  male  and  female  offspring.  However,  crosses  between  individuals 
from  different  races  sometimes  result  in  intersexes  as  well  as  normal  progeny. 
In  these  moths  the  female  is  heterogametic  and  the  male-determining  factors 
seem  to  reside  on  the  Z  chromosomes.  The  female-determining  factors  appear 
to  be  carried  by  the  W  chromosome,  the  autosomes,  and  perhaps  in  the  cytoplasm. 
In  different  races  the  effectiveness  of  these  factors  in  determining  sex  varies,  so 
that  some  races  are  "weak"  and  others  "strong."  For  example,  if  a  "weak"  Euro- 
pean female  is  crossed  to  a  "strong"  Japanese  male,  the  sons  are  normal  but  the 
daughters  intersexual.  The  single  "strong"  Japanese  Z  chromosome  is  sufficient 
to  overcome  the  effects  of  the  female-determining  factors  so  that  the  ZW  indi- 
viduals differentiate  into  intersexes  rather  than  females.  The  F2  from  this  cross 
again  produces  normal  sons,  but  the  daughters  are  half  normal  and  half  inter- 
sexual. The  reciprocal  cross,  "strong"  Japanese  female  with  "weak"  European 
male,  gives  a  normal  Fx,  but  in  the  F2  the  daughters  are  normal,  while  half  the 
sons  are  intersexes  and  half  normal.  Here  again  as  in  Drosophila  a  balance  be- 
tween factors  of  opposite  effect  is  essential  to  normal  sexual  differentiation. 
However,  in  Drosophila  the  intersexes  resulted  from  chromosomal  imbalance.  In 
Lymantria  all  of  the  individuals  are  diploid,  and  the  intersexes  result  from  a 
genie  imbalance.  Therefore,  it  must  be  concluded  that  the  factors  regulating 


EVOLUTION    OF   GENETIC    SYSTEMS  •  313 

normal  sexual  differentiation  have  been  mutually  adjusted  in  the  different  races 
of  the  gypsy  moth  by  many  generations  of  natural  selection. 

This  brief  review  of  sex  determination  and  sexual  differentiation  is  in- 
tended to  show  that  an  individual  is  not  irrevocably  one  sex  or  the  other.  Every 
cell  appears  to  have  the  potential  to  become  either  male  or  female  in  its  charac- 
teristics. The  sex  that  actually  develops  depends  upon  the  type  of  reaction  system 
that  is  set  up  in  the  cell.  If  one  system  is  brought  into  play,  a  male  develops;  the 
other  produces  a  female.  The  factor  determining  which  system  will  prevail  may 
be  environmental,  as  in  Bonellia,  or  it  may  be  genetic,  as  in  the  familiar  chro- 
mosome mechanism  of  sex  determination.  If  the  sex-determining  machinery  itself 
is  thrown  out  of  kilter — for  example,  because  of  chromosomal  imbalance — ab- 
normal sexual  development  will  ensue.  However,  even  if  the  sex-determining 
mechanism  operates  normally,  this  may  not  be  sufficient  to  insure  normal  sexual 
differentiation,  for  unusual  environmental  influences  such  as  hormones,  tempera- 
ture, nutrition,  etc.,  may  modify  differentiation  to  the  extent  that  intersexes  or 
sexually  aberrant  individuals  result. 

The  Control  of  Recombination 

From  an  evolutionary  standpoint  the  separation  of  the  sexes  into  male 
and  female  individuals  may  be  regarded  as  a  means  of  insuring  cross  fertilization 
and  genetic  recombination.  A  comparative  examination  of  the  genetic  systems  in 
numerous  groups  of  plants  and  animals  reveals  a  wide  range  in  the  amount  of 
recombination.  The  available  evidence  suggests  that  recombination  itself  is  under 
the  control  of  natural  selection,  and  that  the  differences  between  groups  in  the 
amount  of  recombination  are  adaptive. 

Numerous  mechanisms  are  known  to  increase  recombination.  Meiosis 
provides  for  a  regular  segregation  and  reassortment  of  the  chromosomes,  and  a 
high  chromosome  number  and  a  high  frequency  of  chiasma  formation  will  also 
increase  the  amount  of  recombination  taking  place.  The  separation  of  the  sexes, 
of  course,  makes  cross  fertilization  mandatory,  but  even  in  hermaphrodites, 
devices  that  reduce  or  prevent  selfing  are  common.  Differences  in  time  of  matura- 
tion of  the  gametes,  or  flower  structures  that  make  self-pollination  unlikely  are 
cases  in  point.  Species  with  reciprocal  cross  fertilization  often  have  the  male  and 
female  reproductive  tracts  completely  separated.  Systems  of  self-sterility  alleles 
also  prevent  self-fertilization  in  many  species.  More  or  less  permanent  hybridity, 
which  appears  in  many  cases  to  take  advantage  of  heterotic  effects,  is  maintained 
by  systems  of  balanced  lethals,  inversion  or  translocation  heterozygotes,  or  by 
allopolyploidy. 

On  the  other  hand,  several  factors  are  known  that  tend  to  reduce  or 
suppress  recombination.  The  organization  of  the  genetic  material  into  linkage 
groups  in  the  chromosomes  prevents  free  recombination  among  genes.  The 
smaller  the  number  of  chromosomes,  the  greater  the  restriction  on  recombination. 


314  •  THE    MECHANISM    OF   EVOLUTION 

Furthermore,  reduction  in  chiasmata  frequency  will  still  further  limit  genie  re- 
combination. Interference  in  regions  adjacent  to  a  chiasma  limits  the  number  of 
crossovers  and  hence  the  amount  of  recombination  possible  within  a  linkage 
group  in  any  one  generation.  Thus,  integrated  gene  complexes  will  not  be  com- 
pletely disrupted  by  crossing  over.  In  Drosophila,  not  only  are  the  chromosome 
numbers  low,  but  crossing  over  is  completely  suppressed  in  the  males  so  that 
recombination  between  homologous  chromosomes  is  possible  only  in  the  females. 
Structural  hybridity  for  inversions  or  translocations  may  effectively  prevent  re- 
combination within  the  affected  chromosome  pairs.  However,  the  cross-over 
frequency  is  often  increased  in  other  chromosome  pairs  in  the  presence  of  a 
structurally  heterozygous  pair.  In  this  way  recombination  within  the  chromosome 
complement  can  be  brought  under  quite  specific  control  by  natural  selection. 

Self-fertilization  will  also,  of  course,  reduce  the  frequency  with  which 
new  gene  combinations  are  formed.  The  effect  of  selfing  is  to  increase  the  fre- 
quency of  homozygotes  in  the  species  population.  The  recessive  mutations  as 
well  as  the  dominants  are  soon  brought  to  expression  and  exposed  to  natural 
selection.  The  elimination  of  the  less  well-adapted  types  results  in  a  loss  of  vari- 
ability, which  is  replenished  only  by  mutation  and  not  by  recombination.  A  self- 
fertilizing  species  then  sacrifices  evolutionary  plasticity  in  favor  of  immediate 
fitness,  and  forms  a  complex  of  relatively  homozygous  individuals  no  longer 
capable  of  gene  exchange.  In  hermaphroditic  species,  a  range  of  conditions  may 
be  found  from  virtually  complete  self-fertilization  to  obligatory  outcrossing. 
The  cross  sterility  observed  in  numerous  instances  is  one  way  in  which  inbreed- 
ing is  enforced.  The  range  of  possibilities  for  breeding  systems  in  hermaphro- 
dites suggests  that  their  modes  of  reproduction  have  been  adaptively  modified. 
In  general,  it  appears  that  the  various  devices  leading  to  selfing  are  of  more 
recent  origin  and  represent  a  method  for  restricting  recombination. 

The  suppression  of  recombination  is  even  more  effective  in  species  re- 
producing asexually.  Asexual  methods  of  reproduction  have  arisen  independently 
in  various  ways  and  in  many  different  groups  of  sexually  reproducing  plants  and 
animals.  Apomixis  is  the  term  used  to  describe  a  variety  of  kinds  of  asexual 
process  in  which  the  outward  appearance  of  sexual  reproduction  is  retained  but 
no  fertilization  occurs.  Parthenogenesis  refers  specifically  to  the  development  of 
unfertilized  eggs.  Asexual  reproduction  in  animals  frequently  occurs  by  means  of 
parthenogenesis,  though  budding  or  fission  is  characteristic  of  certain  groups. 
(Though  often  classified  as  sexual,  parthenogenesis  in  effect  more  nearly  resem- 
bles asexual  reproduction.)  In  plants,  many  additional  types  of  asexual  repro- 
duction are  known:  adventitious  buds,  bulblets,  and  stolons,  in  addition  to  the 
apomictic  formation  of  seeds  not  only  by  parthenogenesis  but  also  from  various 
types  of  somatic  cells.  Many  species  combine  the  advantages  of  sexual  and 
asexual  reproduction.  In  the  aphids,  for  instance,  cyclical  parthenogenesis  per- 
mits a  very  rapid  build-up  in  numbers  during  the  favorable  warm  summer 


EVOLUTION    OF   GENETIC   SYSTEMS  •  315 

months.  Since  every  individual  is  a  female  and  reproduction  is  not  delayed  until 
after  mating,  the  reproductive  potential  of  such  a  population  is  almost  inevitably 
greater  than  that  of  a  population  containing  both  males  and  females.  In  the  fall, 
a  sexual  generation  intervenes,  and  from  the  fertilized  eggs  emerge  the  females 
that  start  the  parthenogenetic  phase  once  again  the  following  spring. 

The  various  types  of  asexual  reproduction  are  similar  to  self-fertilization, 
in  that  groups  of  individuals  of  identical  genotype  are  formed  that  no  longer  are 
capable  of  gene  exchange  with  members  of  other  groups.  They  are  dependent 
upon  mutation  for  further  evolution.  However,  unlike  species  where  selfing  is 
the  rule  and  homozygosity  is  the  norm,  asexual  methods  of  reproduction  ordi- 
narily preserve  the  heterozygosity  intact  from  one  generation  to  the  next.  The 
descendants  of  a  single  individual  will  all  have  the  same  genotype  and  form  a 
clone,  but  this  particular  genotype  may  be  highly  heterozygous.  In  fact,  one 
advantage  of  asexual  reproduction  is  its  preservation  of  heterotic  or  otherwise 
favorable  gene  combinations,  or  of  favorable  chromosome  combinations,  aneu- 
ploid  or  polyploid,  which  are  meiotically  unstable. 

Generally,  the  changes  in  the  genetic  systems  that  result  in  the  restriction 
or  elimination  of  recombination  have  taken  place  in  species  where  immediate 
fitness  and  a  high  reproductive  rate  are  at  a  premium.  There  are  three  major 
mechanisms  that  limit  recombination:  a  reduction  in  chromosome  number  and 
chiasma  formation,  a  shift  toward  self-fertilization,  and  the  development  of 
asexual  methods  of  reproduction.  These  devices,  which  lead  to  similar  results, 
are  apt  to  be  mutually  exclusive.  If  one  type  of  mechanism  prevails  within  a 
group — for  example,  self-fertilization — it  is  unlikely  that  the  others  will  be 
found  to  any  significant  degree  within  the  same  group.  Furthermore,  the  retreat 
from  the  cross  fertilizing,  diploid  condition,  though  it  confers  immediate  adap- 
tive advantage  and  fitness,  does  so  at  the  expense  of  long-range  adaptability.  The 
loss  of  the  flexibility  made  possible  by  genetic  recombination  seems  destined  to 
lead  ultimately  to  the  extinction  of  those  groups  that  travel  too  far  down  this 
path,  for  they  will  be  unable  to  cope  with  or  adapt  to  changing  environmental 
conditions. 

Sexual  Selection 

In  1871  Darwin  published  a  work  entitled  The  descent  of  man  and 
selection  in  relation  to  sex.  In  this  book  he  set  forth  his  opinions  on  the  origin 
and  evolution  of  man,  a  subject  he  had  deliberately  dismissed  with  just  a  sen- 
tence in  The  origin  of  species,  in  the  hope  that  he  would  thereby  not  add  to  the 
prejudices  against  his  views.  Darwin's  writings  on  human  evolution  are  still 
cited  rather  regularly.  However,  the  greater  part  of  this  book  was  actually  de- 
voted to  sexual  selection,  and  his  theories  in  this  area  have  generally  been  either 
rejected  or  ignored.  It  seems  clear  that  he  regarded  the  theory  of  sexual  selection 


316  •  THE    MECHANISM    OF   EVOLUTION 

as  almost  equal  in  importance  to  the  theory  of  natural  selection.  As  he  put  it, 
"Sexual  selection  depends  on  the  success  of  certain  individuals  over  others  of  the 
same  sex,  in  relation  to  the  propagation  of  the  species;  whilst  natural  selection 
depends  on  the  success  of  both  sexes,  at  all  ages,  in  relation  to  the  general  con- 
ditions of  life."  One  reason  the  theory  of  sexual  selection  has  received  so  little 
attention  is  that  it  is  now  realized  that  sexual  selection  is  merely  one  aspect  of 
natural  selection.  Today  natural  selection  is  denned  in  terms  of  reproductive  fit- 
ness. Those  genes  conferring  fitness,  whether  they  contribute  to  survival  or  to 
mating  success,  in  the  final  analysis  tend  to  increase  in  frequency  in  subsequent 
generations  in  much  the  same  way.  Thus,  sexual  selection  is  comparable  in  its 
effects  to  differential  viability,  longevity,  or  fecundity,  and  can  quite  properly 
be  grouped  with  them  as  one  of  the  elements  in  natural  selection. 

A  second  reason  for  the  rejection  of  sexual  selection  is  that  Darwin 
postulated  that  it  came  about  in  two  ways,  through  male  competition  or  through 
female  choice.  These  two  intrasexual  selective  mechanisms  have  been  subject  to 
strong  criticism  ever  since  they  were  first  proposed:  female  choice,  primarily  be- 
cause it  is  anthropomorphic;  male  competition,  because  in  many  species  there  is 
little  evidence  that  the  male  successful  in  competition  with  other  males  neces- 
sarily leaves  more  progeny. 

Nevertheless,  the  phenomena  that  led  Darwin  to  formulate  the  theory 
of  sexual  selection  still  remain,  but  little  progress  has  been  made  toward  a  more 
adequate  theory  or  a  better  understanding  of  the  facts.  The  trend  in  the  evolution 
of  the  higher  animals  has  been  toward  sexually  reproducing  species  with  the 
sexes  separate.  In  most  such  species,  sexual  dimorphism  prevails,  which  in  some 
cases  is  quite  striking.  Darwin's  proposal  was  an  attempt  to  account  for  the 
origin  of  sexual  dimorphism.  As  such,  it  is  undoubtedly  inadequate.  However, 
the  significant  aspect  of  his  theory  is  its  emphasis  on  the  fact  that  the  appearance 
and  behavior  of  individuals  can  influence  the  course  of  evolution  through  their 
effects,  via  the  nervous  system,  upon  other  organisms.  Thus,  the  behavior  and 
appearance  of  an  individual  not  only  affects  its  own  chances  of  survival,  but  also 
influences  the  activity,  behavior,  survival,  and  reproduction  of  other  individuals. 
The  evolution  of  the  nervous  system  thereby  added  a  new  dimension  to  evolu- 
tion. Darwin's  theory  was  inadequate,  not  so  much  because  it  was  wrong,  but 
because  it  was  incomplete.  In  polygamous  species  especially,  male  competition 
may  have  played  a  significant  role  in  the  evolution  of  males  larger  and  better 
equipped  for  combat  than  the  females  (for  example,  in  deer  and  seals).  To  some 
extent,  female  "choice"  may  also  be  significant,  in  the  sense  at  least  that  the 
male  with  the  more  effective  courtship  pattern  will  have  greater  success  in  gain- 
ing the  acceptance  of  the  female  as  a  sexual  partner.  However,  these  possibilities 
are  but  two  among  many  that  could  lead  to  sexual  dimorphism.  The  allesthetic 
traits,  as  they  have  been  called,  which  become  effective  via  the  nervous  systems 
of  other  organisms,  serve  a  variety  of  functions  in  addition  to  sexual  selection. 


EVOLUTION    OF   GENETIC    SYSTEMS  •  317 

Even  with  respect  to  reproduction  these  traits  have  functions  other  than  influenc- 
ing female  choice  or  success  in  male  competition.  For  example,  various  stimuli 
serve  to  bring  the  sexes  together.  Male  moths  are  attracted  to  the  females  over 
considerable  distances  by  their  scent,  which  is  species  specific.  The  calls  of  male 
frogs  and  toads  in  their  breeding  ponds  and  of  male  birds  on  their  nesting  ter- 
ritories are  comparable  in  advertising  their  presence  and  attracting  the  females. 
Furthermore,  the  elaborate  courtship  patterns  involving  a  complex  sequence  of 
stimuli  and  responses  between  male  and  female  serve  for  attraction,  sexual  recog- 
nition, synchronization  of  mating  behavior,  and  arousal  to  the  peak  necessary  for 
the  successful  completion  of  coition.  Even  ovulation  has  been  shown  in  many 
species  to  be  dependent  upon  not  just  hormonal  stimuli  but  on  the  interplay 
between  hormonal  stimuli  and  the  nervous  stimuli  set  off  by  courtship  and 
mating.  Those  traits  in  males  and  females  that  are  epigamic — that  is,  contribute 
to  the  successful  union  of  the  gametes — will  have  adaptive  value  and  will  tend 
to  be  favored  by  selection. 

One  of  the  fundamental  problems  in  the  origin  of  secondary  sexual 
dimorphism  is  genetic  and  developmental.  The  differences  between  males  and 
females  are  known  to  be  due  in  mammals  to  the  influence  of  the  endocrine  sys- 
tem during  development.  In  insects,  cellular  autonomy  exists  with  respect  to 
sexual  differentiation.  Furthermore,  it  is  known  that  the  genotypes  of  males  and 
females  are,  to  a  very  large  extent,  the  same,  for  the  autosomes  are  identical  in 
both  sexes.  The  genetic  differences  may  be  merely  haploidy  versus  diploidy,  one 
X  versus  two  X  chromosomes,  presence  or  absence  of  a  Y;  or,  some  seemingly 
trivial  environmental  difference  may  determine  which  path  sexual  development 
will  follow.  The  problem,  very  simply,  is  to  explain  the  origin  of  the  very  con- 
siderable differences  between  the  sexes  when  the  genetic  differences  between 
males  and  females  are  so  slight.  Sexual  differentiation  is  rather  well  understood, 
for  example,  at  the  level  of  hormonal  control.  The  initiation  and  regulation  of 
sexual  development  under  the  control  of  pituitary  and  gonadal  hormones  has 
been  extensively  studied  experimentally.  However,  at  the  level  of  gene  action,  no 
comparable  knowledge  is  available.  The  nature  of  the  genetic  control  that  brings 
one  developmental  system  into  play  rather  than  the  other  is  not  at  all  well  under- 
stood and  poses  a  particularly  difficult  problem  since  to  a  large  extent  the  same 
genetic  material  is  responsible  in  each  case.  This  area  of  developmental  genetics 
seems  to  hold  problems  of  considerable  interest  from  the  standpoint  of  genetics, 
embryology,  and  evolution. 

In  addition  to  their  epigamic  functions,  the  allesthetic  traits  may  pro- 
mote conspicuousness  or,  quite  the  reverse,  be  cryptic  in  function.  Most  epigamic 
traits,  whether  behavioral  or  morphological,  are  conspicuous,  and  these  same 
traits  may  sometimes  serve  other  functions.  In  threatening  another  male  invading 
his  territory,  for  example,  a  brightly  colored  male  may  use  the  same  colors  in  the 
threat  display  as  he  uses  in  the  courtship  display  before  the  female.  Conspicuous 


318  •  THE    MECHANISM    OF   EVOLUTION 

traits  have  evolved  not  only  in  relation  to  threat  but  also  for  use  as  warning 
signals.  The  various  aspects  of  group  behavior,  too  complex  to  be  detailed  here, 
but  including  care  of  the  young,  colony  and  flock  formation,  cooperation  of 
various  sorts,  and  the  social  behavior  of  insects,  are  built  upon  intricate  and  care- 
fully integrated  systems  of  interactions  among  individuals,  and  are  mediated  by 
the  nervous  system.  These  behavorial  systems  have  emerged  as  a  product  of  evo- 
lution. The  relatively  inflexible  behavior  patterns  that  we  call  instincts  are  clearly 
under  hereditary  control.  The  capacity  to  learn,  also  an  evolutionary  product, 
makes  possible  more  flexible  behavior  patterns  that  can  be  modified  as  the  result 
of  experience. 

Cryptic  behavior  and  form  have  also  resulted  from  the  operation  of 
evolutionary  forces.  The  ability  to  select  a  favorable  habitat  or  resting  place, 
cryptic  behavior  such  as  shadow  elimination,  and  mimicry  and  cryptic  coloration 
— all  have  evolved  as  the  result  of  natual  selection  favoring  those  individuals 
best  able  to  avoid  perception  by  their  enemies.  In  the  light  of  these  few  ex- 
amples, to  which  so  many  more  could  be  added,  there  can  be  little  doubt  that 
Darwin,  in  his  theory  of  sexual  selection,  was  on  the  track  of  a  significant  phase 
of  evolution,  the  psychological  or  ethological  aspect.  The  course  of  evolution  in 
animals  has  been  greatly  influenced  by  the  interactions  that  occur  among  indi- 
viduals and  are  mediated  by  the  sense  organs  and  the  nervous  system.  A  killdeer, 
when  its  nest  is  threatened  by  an  intruder,  dramatically  feigns  injury.  Anyone 
who  has  ever  been  deceived  and  led  astray  by  such  a  display  can  hardly  fail  to 
be  impressed  by  the  subtlety  and  power  of  the  forces  of  evolution. 

SUMMARY  < 

A  major  thesis  of  this  chapter  is  that  not  only  organisms 
but  their  underlying  genetic  systems  have  undergone  evolutionary 
change  and  that  the  genetic  system  itself  may  have  adaptive  value. 
With  few  exceptions  the  hereditary  material  in  living  things  is 
deoxyribonucleic  acid  (DNA).  A  variety  of  methods  of  genetic 
recombination  have  been  discovered,  from  the  novel  types  de- 
scribed in  viruses  and  bacteria  to  the  orderly  system  in  higher 
plants  and  animals.  This  orderliness  became  possible  with  the 
organization  of  the  genes  into  chromosomes  that  undergo  regular 
mitotic  and  meiotic  cell  divisions.  Asexual  reproduction  is  espe- 
cially well  suited  to  the  rapid  self-duplication  of  a  particular 
genotype,  and  thus  is  favorable  to  the  maintenance  of  a  well- 
adapted  genotype  in  a  stable  environment  or  to  rapid  colonization. 
A  sexually  reproducing  population,  on  the  other  hand,  is  better 
able  to  adjust  to  changing  environmental  conditions  and  to  ex- 
ploit new  and  different  ecological  niches.  Haploidy  is  the  more 
primitive  condition,   whereas  the  predominance  of  the  diploid 


EVOLUTION   OF   GENETIC   SYSTEMS  •  319 

generation  is  associated  with  the  evolution  of  organisms  of  con- 
siderable complexity.  The  evolution  of  sex  has  led  to  the  evolu- 
tion of  numerous  methods  for  controlling  sex  determination  and 
sexual  differentiation.  Sexual  anomalies  may  result  when  either  of 
these  processes  is  disrupted.  In  sexually  reproducing  species,  the 
amount  of  genetic  recombination  is  regulated  in  a  variety  of  ways, 
which  range  from  self -sterility  or  enforced  outcrossing  to  self- 
fertilization.  The  release  of  genetic  variability  appears  to  be  under 
rather  precise  control.  Darwin's  theory  of  sexual  selection,  though 
inadequate  in  many  respects,  seems  to  merit  further  study,  for  it 
focuses  attention  on  the  fact  that  the  appearance  and  behavior  of 
an  individual  not  only  affects  its  own  chances  of  survival  but  also 
influences  the  activity  and  behavior,  survival  and  reproduction  of 
other  individuals  as  well. 

SUGGESTED  READING 

Darlington,  C.  D.,  1958.  The  evolution  of  genetic  systems.  New  York:  Basic  Books. 

Stebbins,  G.  L.,  I960.  "The  comparative  evolution  of  genetic  systems,"  Evolution 
after  Darwin,  Vol.  1,  The  evolution  of  life.  Chicago:  University  of  Chi- 
cago Press. 


f 


ww% 


PART 


IV 


Evolution 
and  Man 


CHAPTER 


33 

Human  Evolution 


The  Mammalia  are  a  class  of  vertebrates  or  back-boned 
animals  characterized  by  mammary  glands,  hair,  and  body  temper- 
ature regulation.  The  subclass  Eutheria,  or  placental  mammals, 
bear  living  young  that  undergo  a  period  of  development  within 
the  uterus  of  the  female.  The  Primates  are  placental  mammals 
with  elongated  limbs  and  enlarged  hands  and  feet,  each  with  five 
digits.  The  digits  have  nails  rather  than  claws  or  hoofs,  and  the 
thumb  and  the  great  toe  are  usually  opposable  to  the  other  digits. 
Primates  are  generally  arboreal  and  are  found  primarily  in  tropical 
and  subtropical  regions.  Their  orbits  are  directed  forward  so  that 
they  have  binocular  vision.  Except  for  the  highly  developed  brain 
and  nervous  system,  the  Primates  are  a  relatively  generalized 
group.  Any  objective  analysis  of  human  traits  will  lead  inevitably 
to  the  conclusion  that  man  is  a  vertebrate,  a  placental  mammal, 
and  a  primate.  He  differs  from  other  primates  primarily  in  his 
enlarged  brain  and  erect  posture.  He  is  cosmopolitan  rather  than 
tropical,  terrestrial  rather  than  arboreal,  and  the  great  toe  is  not 
opposable.  His  mastery  of  the  arts  of  making  fire  and  clothing 
first  permitted  him  to  extend  his  range  beyond  the  tropics,  and 
without  these  he  would  once  again  be  a  tropical  species.  The  un- 
usual size  of  the  great  toe  and  shape  of  the  foot  are  clear  indica- 
tions of  his  ancestors'  descent  from  trees  in  the  not  too  remote  past. 
The  Primates  have  been  classified  as  shown  in  Table  33-1. 

The  Prosimians 

The  most  primitive,  generalized  mammals  such  as  shrews 
and  moles  belong  to  the  order  Insectivora,  from  which  all  other 


323 


324  •  EVOLUTION    AND    MAN 


TABLE    33-1 
The  Primates 


Suborder         Superfamily         Family 


Common 
name 


Distribution 


Remarks 


orders  of  mammals  are  thought  to  have  descended.  For  many  years  the  tree 
shrews  were  included  among  the  insectivores.  More  recently,  however,  they  have 
been  grouped  with  the  primates,  for  even  though  conforming  to  the  basic  mam- 
malian plan,  they  show  in  their  slightly  enlarged  brains  and  eyes  the  beginnings 
of  primate  traits.  Superficially,  the  tree  shrews  resemble  squirrels,  for  they  are 
small,  bushy-tailed  animals  that  are  active  by  day.  They  possess  claws  rather  than 
nails,  but  their  simple  incisor  teeth  are  quite  different  from  those  of  the  squirrels, 
which  are  typical  of  the  chisellike  gnawing  incisors  of  the  rodents.  Their  digits, 
their  eyes,  and  their  brain  separate  them  from  the  insectivores  and  place  them 
with  Primates.  Thus  the  Primates,  the  order  to  which  man  belongs,  are  linked 
directly  through  the  tree  shrews  to  the  oldest  group  of  mammals.  Certainly  in 
this  instance,  there  is  no  reason  to  speak  of  a  "missing  link." 

The  true  lemurs  and  the  aberrant  aye-aye  are  found  now  only  on  the 
island  of  Madagascar,  but  formerly  they  ranged  over  much  of  the  Old  World 
and  North  America.  About  the  size  of  a  mouse  or  a  cat,  the  lemurs  are  usually 
both  arboreal  and  nocturnal.  Although  they  display  primate  characteristics,  they 
are  rather  foxlike  in  appearance  due  to  their  elongated,  moist  muzzles  and 
rather  large,  mobile  ears.  Their  brains,  compared  to  those  of  monkeys  or  men, 
are  relatively  simple,  for  the  cerebral  cortex  is  small  and  smooth,  lacking  the 
folds  that  greatly  increase  the  surface  area  in  the  higher  primates. 


Prosimii 

Tupaioidea 

Tree  shrews 

Oriental 

6  genera.  Mod- 

lower 

erate    number 

Primates 

of  species 

Lemuroidea 

Lemurs 

Madagascar 

19  species 

Daubento- 

Aye-Aye 

Madagascar 

1  species 

nioidea 

Lorisformes 

Loris,  galagos, 
bush  babies, 

Africa  and 
Oriental 

10  species 

Tarsi  if  ormes 

pottos 
Tarsiers 

East  Indies 

3  species 

Anthropoidea 

Ceboidea          Cebidae 

New  World 

New  World 

12  genera, 

higher 

monkeys 

tropics 

140  species 

Primates 

Cal 

Marmosets 

2  genera 

thricidae 

several  species 

Cercopi-           Cercopi- 

Old  World 

Old  World 

16  genera, 

thecoidea 

thecidae 

monkeys 

tropics  except 
Australia 
Old  World 

200  species 

'Pongidae 

Apes 

10  species 

tropics  except 

Hominoidea 

Australia 

Hominidae 

Man 

Cosmopolitan 

1  species, 
Homo  sapiens 

HUMAN    EVOLUTION  •  325 

The  Lorisiformes  include  species  with  such  appealing  names  as  bush 
baby  and  potto,  and  are  in  general  rather  like  the  lemurs.  The  lorises  of  Asia  are 
slow-moving  climbers  with  relatively  large  eyes  and  a  shorter  snout  than  most  of 
the  true  lemurs.  The  galagos  or  bush  babies  native  to  Africa  are  small  and  active, 
with  their  hind  legs  specially  adapted  for  jumping. 

The  tiny  tarsiers,  the  size  of  small  kittens,  though  formerly  found  in 
much  of  the  Old  World  and  North  America,  today  live  only  in  the  East  Indies. 
They  have  an  unusual  combination  of  primitive  characters  that  link  them  to  the 
lemurs,  and  advanced  traits  that  suggest  relationship  to  the  monkeys.  The  tarsiers 
have  a  short  face  with  relatively  enormous  eyes  facing  to  the  front,  undoubtedly 
an  adaptation  to  their  nocturnal,  arboreal  habits.  Their  limbs  and  feet  are  spe- 
cially modified  for  both  grasping  and  jumping,  so  that  they  flit  through  the 
trees  with  surprising  ease.  The  tarsier's  large  brain,  well-developed  senses  of 
vision  and  hearing,  and  the  structure  of  nose  and  lips  all  suggest  relationship  to 
the  monkeys,  but  his  fossil  relatives  show  him  to  be  more  closely  related  to  the 
lemurs. 

It  is  of  particular  interest  that  within  the  rather  heterogeneous  sub- 
order Prosimii,  the  animals  range  in  kind  from  the  tree  shrews,  which  are  not 
far  removed  from  the  most  primitive  placental  mammals,  the  insectivores,  to  the 
tarsiers,  which  foreshadow  the  monkeys  and  the  other  Anthropoidea  (see  Fig. 
33-1). 

The  Higher  Primates 

The  higher  primates,  including  the  monkeys,  apes,  and  man,  belong  to 
the  suborder  Anthropoidea.  Though  called  "higher,"  there  is  not  much  that  is 
strikingly  different  about  them  as  compared  to  the  lower  primates.  The  differ- 
ences, however,  are  of  considerable  significance.  In  particular,  their  eyes  show 
several  changes  that  permit  superb  vision.  The  yellow  spot,  or  macula  lutea,  in 
the  retina  directly  opposite  the  pupil  is  a  region  of  especially  acute  sight. 
Furthermore,  the  color  vision  of  the  Anthropoidea  is  superior  to  that  of  any  of 
the  other  mammals.  The  placement  of  the  eyes,  in  sockets  facing  directly  for- 
ward, permits  both  eyes  to  cover  the  same  field  of  vision.  This  arrangement 
differs  greatly  from  that  of  a  deer,  for  example,  where  each  eye  has  a  separate 
field  of  vision  with  relatively  little  overlap.  The  higher  primates  are  thus  able  to 
see  not  only  clearly  and  in  color  but  also  in  three  dimensions.  The  effect  of 
binocular  vision  is  similar  to  that  of  an  old-fashioned  stereopticon,  for  each  eye 
views  an  object  from  a  slightly  different  direction,  and  the  object  seems  to  stand 
out  in  three  dimensions  so  that  very  accurate  estimates  of  distance  are  possible. 
In  contrast  to  the  nocturnal  prosimians,  the  higher  primates  are  active  by  day. 
They  are  also  typically  larger  than  the  lower  primates.  Incidentally,  even  though 
man  is  often  pictured  as  a  weak,  defenseless  creature,  in  reality  even  without 


326  •  EVOLUTION    AND    MAN 


:i   b 


\ 


■■■■■ 


Fig.  33-1.  Representative  prosimians.  (a) 
Tree  shrew  (Tupaia  minor);  (b)  Mindanac 
tarsier  (Tarsius  carbonarins);  (c)  Galagc 
(Galago  crassicaudatHs);  (d)  Aye-aye  (Dam 
bentonia  madagascariensis);  (e,  left)  Mous( 
lemur  (Ai/crocebus  murinis).  (With  permis 
sion  of  Zoological  Society  of  London  [a,  d 
and  e],  Walker  [b]  and  Chicago  Zoologica 
Park  [c].) 


modern  weapons  he  is  a  rather  formidable  animal,  as  are  the  orangutan,  chim- 
panzee, and  gorilla.  In  addition  to  improved  vision,  the  most  striking  difference 
between  higher  and  lower  primates  lies  in  the  larger  brain  of  the  former,  with 


HUMAN    EVOLUTION  •  327 

the  cerebral  cortex  assuming  ever-greater  importance.  The  cerebrum,  where  the 
higher  mental  functions  are  localized,  covers  more  and  more  of  the  brain  until 
in  man  it  virtually  overlies  the  rest  of  the  brain. 

The  Anthropoidea  have  been  divided  into  two  major  groups,  the  platyr- 
rhines  of  the  Americas,  including  the  New  World  monkeys  and  marmosets,  and 
the  Old  World  catarrhines,  including  men,  apes,  and  the  monkeys  of  the  Old 
World  tropics.  The  platyrrhines  are  flat  nosed,  with  the  nostrils  widely  spaced. 
In  the  catarrhines  the  nostrils  are  close  together  and  point  downward. 

In  addition  to  their  noses,  perhaps  the  most  striking  trait  of  the  New 
World  monkeys  is  their  prehensile  tail  by  which  most  of  them  can  hang  or  swing 
from  branches  or  use  as  a  fifth  hand.  The  little  marmosets  scarcely  look  like 
monkeys,  for  they  have  claws  rather  than  nails  (except  on  the  big  toe)  and  their 
thumbs  are  not  opposable.  Furthermore,  some  of  them  have  manes,  and  their  fur 
typically  has  a  banded  pattern. 

The  monkeys  of  the  Old  World  lack  prehensile  tails  and  some  species 
such  as  the  baboons  have  become  terrestrial,  living  in  rocky,  open  country.  As  a 
group,  the  Cercopithecidae  are  more  generalized  in  body  form  than  the  New 
World  monkeys,  and  are  not  so  completely  adapted  to  arboreal  life.  Their  hands 
look  rather  human,  for  the  thumbs  have  good  opposability.  The  Old  and  New 
World  monkeys  differ  not  only  in  their  distribution  and  the  traits  just  mentioned 
but  also  in  such  fundamental  anatomical  traits  as  dentition  and  structure  of  the 
skull.  See  Fig.  33-2. 

The  two  remaining  groups  of  catarrhines,  because  of  their  similarities, 
have  been  placed  in  a  single  superfamily,  the  Hominoidea.  These  two  groups  are 
the  anthropoid  apes  of  the  family  Pongidae  and  the  family  to  which  man  himself 
belongs,  the  Hominidae.  The  living  anthropoids  are  the  gibbons,  the  orangutans, 
the  chimpanzees,  and  the  gorillas.  Man  and  the  apes  show  many  more  similarities 
than  man  and  the  monkeys.  Not  only  are  they  large  in  size  and  lacking  a  tail, 
but  in  many  fundamental  morphological  and  physiological  traits  they  are  much 
alike.  In  the  details  of  their  brain  and  skull,  dentition,  and  skeleton  they  show 
rather  close  affinities.  Many  of  these  resemblances  result  from  the  adoption  of  an 
erect  posture  with  the  associated  changes  in  such  traits  as  the  shape  of  the  chest, 
the  position  of  the  abdominal  organs,  and  the  shape  of  the  pelvis.  Furthermore, 
in  such  matters  as  reproductive  physiology,  blood  group  chemistry,  and  even 
susceptibility  to  parasites,  they  show  evidence  of  rather  close  genetic  ties.  The 
main  differences  between  man  and  the  apes  are  associated  with  their  modes  of 
locomotion,  for  the  apes  are  essentially  brachiators,  swinging  upright  through  the 
trees  by  their  hands,  while  man  walks  erect  on  the  ground.  The  gibbons,  superb 
aerialists,  live  in  the  tropical  forests  of  Southeast  Asia.  The  orangutan  also  lives 
in  this  region  but  is  now  confined  to  the  islands  of  Borneo  and  Sumatra.  Orangs, 
like  the  gibbons,  are  completely  arboreal,  but  since  they  are  much  larger  than 
gibbons,  they  are  comparatively  slow  moving  and  deliberate  in  their  actions.  The 


328  •  EVOLUTION    AND    MAN 


Fig.  33-2.  Representative  monkeys.  New 
World:  (a)  Humboldt's  woolly  monkey 
{Lagothrix  lagotricha);  (b)  Lion-headed 
or  golden  marmoset  (Leontocebus  rosalia). 
Old  World:  (c)  Pig-tailed  macaque  (Ma- 
caca  nemestrina).  (With  permission  of 
Walker  [a,  b]  and  National  Zoological 
Park,  Smithsonian  Institution  [c].) 


HUMAN    EVOLUTION  •   329 


"-      % 


Fig.  33-3.     The  anthropoid  apes,    (a)  Gibbon  (Hylobates);  (b)  Orangutan  (Pon- 
go);  (c)  Gorilla  (Gorilla);  (d)  Chimpanzee  (Pan). 


other  two  anthropoids,  the  gorilla  and  the  chimpanzee,  inhabit  the  tropical 
forests  of  west  central  Africa.  However,  they  are  not  as  strictly  arboreal  as  the 
Asiatic  apes,  for  they  spend  a  considerable  part  of  their  time  on  the  ground. 
Nevertheless,  their  body  structure  is  still  essentially  that  of  a  brachiator  though 
not  so  completely  specialized  for  this  mode  of  life  as  the  gibbon  or  orangutan. 
See  Fig.  33-3. 

Man  is  not  only  a  Primate,  he  is  an  Old  World  catarrhine  and  even 
more  specifically,  his  anatomy  shows  him  to  be  a  hominoid,  a  member  of  the 


330  •  EVOLUTION   AND   MAN 

same  superfamily  as  the  great  apes.  The  hominid  traits,  which  set  him  apart  from 
the  apes,  are  his  feet  and  legs,  which  enable  him  to  walk  erect  on  the  ground, 
with  his  hands  free  for  tasks  other  than  locomotion  (see  Fig.  33-4).  Man's  skull 


Fig.  33-4.     The  upstart. 

and  brain  also  set  him  apart  from  the  apes,  but  these  are  apparently  differences  in 
emphasis  rather  than  in  basic  structure,  and  moreover  they  arose  after  the  differ- 
ences in  leg  structure  had  evolved.  Aside  from  his  obviously  larger  brain,  man's 
head  differs  from  the  apes'  in  that  the  face  is  reduced  in  size  and  has  shrunk  back 
under  the  forehead.  This  recession  of  the  face  appears  related  to  the  better 
balance  of  the  skull  on  the  spine  achieved  by  man  as  compared  to  the  apes. 
Associated  with  this  change  has  been  a  reduction  in  the  size  of  the  teeth  and 
jaws  and  the  emergence  of  the  distinctive  human  nose  and  chin  (see  Fig.  33-5). 


Fossil  Primates 

The  actual  fossil  record  of  the  Primates  is  fragmentary  and  in  many 
ways  unsatisfactory.  However,  it  does  suffice  to  show  that  the  Primates  are  one 
of  the  oldest  orders  of  mammals,  having  a  fossil  record  extending  well  back  into 
the  Mesozoic.  By  the  Paleocene  at  the  beginning  of  the  Tertiary  a  number  of 
prosimians,  such  as  lemurs,  lorises,  and  tarsiers,  were  present  in  relative  abun- 
dance over  most  of  the  world.  After  flourishing  during  the  Paleocene  and 
Eocene,  the  prosimians  vanished  completelv  from  the  Oligocene  in  North  Amer- 
ica and  Europe  and  were  reduced  in  numbers  in  Asia  and  Africa.  The  reasons  for 
their  decline  are  not  known,  but  the  prosimian  hard  times  coincided  not  only 


HUMAN    EVOLUTION  •  331 


Chimpanzee 


Java  man 


African  ape  -  man 
(Australopithecus) 


Neanderthal  man  Modern  man 

Fig.  33-5.     Five  hominid  skulls  shown  with  that  of  an  anthropoid  ape  for  comparison. 


332  •  EVOLUTION   AND   MAN 

with  the  rise  of  such  mammalian  groups  as  the  carnivores  and  rodents  but  also 
with  the  appearance  of  the  higher  primates,  fossils  of  which  first  appear  in  the 
early  Oligocene.  It  seems  a  fairly  safe  assumption  that  competition  from  these 
highly  successful  groups  played  a  significant  role  in  the  decline  of  the  lower 
primates.  On  the  island  of  Madagascar,  which  the  lemurs  reached  but  the  higher 
primates  did  not,  the  lemurs  continued  to  survive  and  evolve,  long  after  they 
became  extinct  elsewhere  in  the  world. 

Fossils  that  suggest  the  hominoid  line  leading  to  the  apes  and  man  also 
appear  at  about  this  time,  some  fifty  million  years  ago.  More  is  known  of  the 
fossil  precursors  of  the  gibbons  than  of  the  other  hominoids.  Propliopithecus 
from  the  Oligocene  of  Egypt  some  35  million  years  ago,  Limnopithecus  from 
East  Africa,  in  the  Miocene  some  10  million  years  later,  and  Pliopithecus  from 
the  late  Miocene  and  early  Pliocene  in  Europe  lead  quite  clearly  up  to  the 
modern  gibbon,  Hylobates.  The  gibbons  thus  were  separated  quite  early  from 
the  other  lines  of  hominoids.  Furthermore,  they  were  not  as  specialized  for 
brachiation  as  the  modern  gibbons,  but  had  more  generalized  limbs  for  climbing. 

Only  in  the  Miocene  do  the  forerunners  of  the  other  apes  begin  to  ap- 
pear in  the  fossil  genera  Dryopithecus  and  Sivapithecus,  found  rather  abundantly 
in  Europe,  Asia,  and  Africa.  The  remains  consist  almost  exclusively  of  jaws  and 
teeth,  so  that  little  is  known  about  whether  they  were  brachiating  animals.  How- 
ever, the  teeth  and  jaws  seem  clearly  to  be  of  a  type  that  today  are  found  in 
modified  form  in  the  great  apes  (gorilla,  chimpanzee,  and  orangutan)  and  in 
man.  Fossils  of  an  even  earlier  type  of  ape  known  as  Proconsul  have  been  found 
in  relative  abundance  in  lower  Miocene  deposits  in  East  Africa.  Although 
Proconsul  has  been  thought  to  be  a  forerunner  of  the  modern  chimpanzee,  in 
reality  he  shows  differences  from  all  other  hominoids,  the  gibbons,  the  great 
apes,  and  man,  as  well  as  from  the  fossil  Dryopithecus  group,  and  he  probably 
represents  a  separate  evolutionary  line.  A  significant  point  brought  out  by  the 
fossil  record  of  the  apes  is  that  the  living  populations  of  apes  are  essentially 
relict  populations;  the  fossil  apes  were  evidently  much  more  widespread  and 
abundant  than  are  the  living  groups. 

The  Fossil  Record  of  Man 

The  existing  races  of  man  are  descended  from  other  somewhat  different 
populations  that  lived  in  the  past.  The  evidence  for  human  evolution  comes  from 
the  fossil  record.  If  man  has  evolved,  then  it  is  necessary  to  try  to  define  the 
stage  in  his  evolution  when  he  first  became  human.  This  stage  can  be  defined  as 
having  been  reached  when  man's  ancestors  became  intelligent  enough  to  make 
tools.  His  ancestors  between  the  ape  and  human  levels  can  then  be  termed  pre- 
human. Though  arbitrary,  this  definition  is  essentially  objective.  The  fossil 
record  of  man  or  preman  is  largely  confined  to  the  Pleistocene,  and  our  knowl- 


HUMAN    EVOLUTION  -333 

edge,  therefore,  covers  primarily  the  last  million  years  of  human  evolution.  Prior 
to  that  time  there  is  a  gap  of  several  million  years  in  the  actual  fossil  record. 

The  fossil  evidence  indicates  that  the  prehumans  must  have  come  from 
a  generalized  anthropoid  ape,  which  lived  on  the  ground  but  had  arboreal  ances- 
tors. The  major  evolutionary  change  leading  to  man  was  the  shift  to  bipedal 
locomotion.  This  change  led  to  changes  in  the  bones  and  muscles  of  the  pelvis, 
legs,  and  feet,  including  the  realignment  of  the  big  toe,  and  in  the  angle  of 
attachment  of  the  skull  to  the  spine.  The  net  effect  enables  man  to  keep  his  body 
erect,  not  by  muscular  action  as  the  apes  do,  but  on  a  bony  supporting  column. 
The  erect  posture  freed  the  hands  from  use  for  locomotion.  The  ancestral 
hominids  were  probably  omnivores  who  shifted  toward  carnivorous  ways  and 
developed  systematic  hunting  habits,  for  the  earliest  known  men  were  hunters 
using  weapons  to  kill  game.  It  is  probable  that  sticks  and  stones  were  used  first, 
and  that  this,  in  turn,  led  to  weapon  making.  The  fossils  indicate  that  much  of 
the  increase  in  brain  size  came  later  during  the  Pleistocene  after  the  shift  to 
bipedal  locomotion. 

Man  and  his  tools  appear  first  in  the  major  warmer  parts  of  the  Old 
World — that  is,  in  Africa  and  southern  Eurasia — and  presumably  prehuman  evo- 
lution occurred  somewhere  in  this  area.  No  living  or  fossil  apes  or  prehumans 
are  found  either  in  America  or  in  the  Australian  region,  a  fact  that  would  also 
seem  to  rule  out  northern  Eurasia  as  the  place  of  man's  origin,  for  expansion  to 
North  America  via  the  Bering  land  bridge  would  then  have  been  a  simple 
matter,  as  it  was  for  many  other  species.  The  prehumans  presumably  evolved  in 
open  country  where  running  on  two  legs  was  an  advantage  to  incipient  hunters. 
Such  a  setting  points  to  Africa,  where  there  was  much  open  country  and  much 
game  to  run  after.  Whatever  the  place,  they  then  dispersed,  as  other  dominant 
successful  groups  of  animals  have  done,  in  a  complex  fashion  over  the  warmer 
parts  of  the  Old  World  during  the  Pliocene.  The  first  known  tools  of  worked 
stone,  from  Olduvai  Gorge  in  Tanganyika,  are  estimated  to  have  been  made 
1,750,000  years  ago,  or  only  70,000  generations  ago. 

During  the  previous  70  million  years  or  so  in  the  Tertiary  the  climate 
of  the  earth  was  rather  warm  and  stable.  With  the  beginning  of  the  Pleistocene 
about  a  million  years  ago,  the  earth's  climate  became  changeable  and  a  period  of 
cooling  was  followed  by  four  major  ice  ages,  with  intervening  warmer  periods. 
Four  times,  tremendous  continental  glaciers  pushed  their  way  down  into  the 
more  temperate  regions,  covering  major  portions  of  Europe,  North  America,  and 
parts  of  Asia.  The  glacial  stages  were  followed  by  warmer  interglacial  stages 
during  which  the  weather  became  even  warmer  than  at  present.  Thus  the  Pleisto- 
cene was  a  time  of  fluctuating,  unsettled  climatic  conditions.  The  stages  have 
been  most  carefully  studied  in  Europe  and  North  America,  and  their  names  and 
approximate  durations  are  shown  in  Table  33-2  together  with  relevant  informa- 
tion about  fossil  man. 


334  •  EVOLUTION   AND   MAN 


TABLE    3  3-2.     Fossil  Men 


Glacial  and  interglacial 

stages 
(estimated  years  ago) 


Cultural  period 


Fossil  hominids 


Postglacial 


10,000 

IV  Wisconsin 

or 
Wiirm  Glacial 


80,000 


Third  Interglacial 


150,000 

III  Illinoian 
or 

Riss  Glacial 
225,000 

Second  or 

"Great"  Interglacial 

350,000 

II  Kansan  or 
Mindel  Glacial 
425,000 


First  Interglacial 


500,000 

I  Nebraskan  or 

Giinz  Glacial 
625,000 

Possible  earlier  glacials 

1,750,000 


Iron 
Bronze 
Neolithic 
Mesolithic 


Upper  Paleolithic 


Middle  Paleolithic 


Lower  Paleolithic 


Cro-Magnon 


Florisbad 
Combe-Capelle 

Neanderthal 
Solo 

Rhodesian 
Mount  Carmel 

Neanderthal 
Fontechevade 

Kanjera 
Rabat-Casablanca 


Steinheim 
Swanscombe 

Peking 

Java 

Ternifine 

Heidelberg 

Telanthropus 

Paranthropus 

Java 

Meganthropus 

Australopithecus 

Kanam  (?) 

Australopithecus 

Zinjanthropus 

Unnamed  Olduvai  hominid 


The  tendency  to  assign  each  newly  discovered  fossil  member  of  the 
Hominidae  to  a  separate  genus  has  led  to  a  confusing  welter  of  names.  Rather 
than  bringing  out  the  similarities  and  differences  among  these  fossils,  the  system 
has  obscured  their  relationships.  Use  of  the  same  taxonomic  criteria  for  hominids 


HUMAN    EVOLUTION  -335 

as  for  other  groups  would  considerably  reduce  the  number  of  genera  and  species. 

Although  the  fossil  record  of  man  is  largely  confined  to  the  last  million 
years  (the  Pleistocene) ,  a  recent  restudy  of  a  fossil  known  as  Oreopithecus  from 
the  lower  Pliocene  in  Italy  some  ten  million  years  ago  may  carry  our  knowledge 
further  back  in  time.  When  found  a  century  ago,  he  was  assigned  to  the  Old 
World  monkeys  or  Cercopithecidae  and  was  more  or  less  ignored.  However, 
further  finds  and  restudy  in  the  light  of  modern  knowledge  have  shown  that 
Oreopithecus  clearly  is  neither  a  monkey  nor  a  Proconsul  nor  a  Dryopithecus.  If 
he  must  be  assigned  to  one  of  the  three  hominoid  groups  (gibbons,  great  apes, 
hominids),  he  comes  closer  to  being  a  hominid  than  anything  else.  This  is  not  to 
say  that  he  is  necessarily  a  direct  ancestor  of  man,  but  rather  that  he  probably 
belonged  some  ten  million  years  ago  to  the  same  group  of  related  species  that 
included  the  ancestors  of  man. 

The  most  primitive  kind  of  fossils  that  are  clearly  those  of  Hominidae 
come  from  deposits  in  South  Africa.  Dr.  Raymond  Dart,  the  anatomist  who 
made  the  original  discovery,  called  his  find  Australopithecus  and  pointed  out  its 
human  characteristics.  This  conclusion  was  at  first  widely  doubted  and  challenged 
by  many  of  the  recognized  authorities.  However,  further  discoveries  by  Broom, 
Robinson,  Leakey,  and  others  have  shown  almost  beyond  question  that  Australo- 
pithecus was  indeed  an  early  hominid  and  not  simply  an  anthropoid  ape  with 
some  slightly  human  traits.  These  additional  fossils,  as  is  customary,  have  been 
given  separate  generic  names  (for  example,  Paranthropus,  Plesianthropus,  Tel- 
anthropus,  and  Zinjanthropus),  but  all  are  enough  alike  (except  perhaps  Telan- 
thropus)  to  be  put  in  the  same  subfamily,  the  Australopithecinae.  Further  mate- 
rial and  additional  study  may  in  time  lead  to  taxonomic  revision  toward  greater 
simplicity.  However,  the  fossils  fall  into  two  main  groups,  typified  by  Australo- 
pithecus and  Paranthropus.  The  Australopithecus  type  was  rather  small,  probably 
weighing  no  more  than  50  or  60  pounds;  Paranthropus  was  considerably  larger 
and  heavier.  The  most  significant  features  of  these  australopithecine  "ape-men" 
were  their  rather  small  brains,  with  a  cranial  capacity  of  about  600  cc — not  much 
greater  than  that  of  a  gorilla  or  a  chimpanzee — associated  with  pelvic  and  leg 
bones  very  similar  to  those  of  modern  man.  Thus,  it  seems  that  erect  bipedal 
locomotion  on  the  ground — in  other  words,  walking  erect — evolved  first  in  the 
human  line  and  that  the  increase  in  size  and  capability  of  the  human  brain 
evolved  later.  This  conclusion  is  contrary'  to  what  was  long  believed  to  be  the 
case,  that  man  was  an  intelligent  ape  who  climbed  down  from  the  trees  to  take 
up  his  abode  on  the  ground.  Furthermore,  the  "ape-men"  had  relatively  massive 
jaws,  but  the  details  of  the  jaws  and  of  the  dentition  were  fundamentally  human 
and  not  apelike  at  all,  and  the  skull,  despite  the  small  size  of  the  brain,  was  of 
the  human  pattern.  Finally,  evidence  has  been  accumulating,  climaxed  by  the 
recent  discovery  by  the  Leakeys  of  a  new  fossil  hominid  that  they  called  Zinjan- 
thropus and  another  hominid  as  yet  unnamed,  that  the  Australopithecinae  were 


336  •  EVOLUTION   AND    MAN 

already  capable  of  using  and  even  fabricating  simple  stone  and  possibly  bone 
tools  some  1,750,000  years  ago.  This  discovery  has  again  required  a  considerable 
revision  in  our  thinking  about  the  course  of  human  evolution,  If  we  are  to  define 
as  "human"  those  hominids  who  could  make  tools,  then  the  terms  "man-apes" 
and  even  "ape-men"  seem  to  be  inappropriate  for  the  Australopithecinae.  Knowl- 
edge of  this  group  has  greatly  increased  our  information  about  the  course  of  evo- 
lution in  the  Hominidae  and  makes  even  more  urgent  the  need  to  find  additional 
early  hominid  remains. 

A  more  advanced  stage  of  human  evolution  is  represented  by  the  fossils 
first  found  by  Dubois  and  known  as  the  Java  man.  They  illustrate  very  nicely  the 
taxonomic  problems  in  paleoanthropology.  Dubois  christened  his  find  Pithecan- 
thropus erectus  and  placed  him  in  a  new  family,  Pithecanthropidae,  between  the 
Pongidae  and  the  Hominidae.  The  quite  similar  Peking  man  was  originally 
placed  in  a  new  genus  and  species,  Sinanthropus  pekinensis.  However,  the  simi- 
larities have  led  some  authorities  to  regard  them  as  two  species  in  the  same 
genus,  Pithecanthropus  erectus  and  P.  pekinensis,  and  Mayr  has  argued  that  by 
the  usual  taxonomic  criteria  they  are  merely  different  races  of  the  same  species 
and  that  this  species  is  sufficiently  like  modern  man  to  be  placed  in  the  same 
genus,  Homo.  Thus  Java  man  would  become  Homo  erectus  erectus  and  Peking 
man  H.  e.  pekinensis.  These  differences  over  nomenclature  may  seem  to  be 
trivial,  but  the  implications  of  each  system  are  quite  different.  It  now  appears 
certain  that  the  two  finds  belong  to  the  Hominidae  rather  than  to  a  separate 
family  and  that  they  belong  together  in  the  same  genus.  Since  their  cranial  capac- 
ities were  quite  different,  they  differed  more  than  the  living  human  races,  and 
thus  the  best  course  may  be  to  take  the  middle  ground  and  consider  them  as 
separate  species  within  the  same  genus,  Pithecanthropus.  The  Java  and  Peking 
men,  living  perhaps  half  a  million  years  ago,  were  hunters  with  stone  tools  who 
lived  in  caves  and  used  fire.  They  had  thick  skulls  with  heavy  brow  ridges,  a 
prognathous  profile  with  large  teeth  but  no  chin,  and  a  cranial  capacity  of  ap- 
proximately 750  to  900  cc  in  Java  man  and  900  to  1200  cc  in  Peking  man.  The 
rest  of  their  skeleton  did  not  differ  from  that  of  modern  man.  One  habit  of  these 
early  humans  is  clearly  recorded.  They  picked  each  others'  brains  and  tossed  the 
skulls  aside  in  their  caves,  there  to  be  discovered  thousands  of  years  later  as  evi- 
dence of  their  cannibalism. 

Another  stage  in  the  evolution  of  man  represented  by  abundant  fossils 
is  known  as  Neanderthal  man,  after  the  valley  in  Germany  where  the  first  care- 
fully studied  fossils  of  this  type  were  discovered  in  1856.  Numerous  fossils  of 
Neanderthal  men  (and  here  we  are  clearly  dealing  with  members  of  the  genus 
Homo)  were  found  in  North  Africa,  in  western  Asia,  and  over  most  of  Europe 
except  Britain  and  the  northern  regions.  They  persisted  for  about  a  hun- 
dred thousand  years,  first  appearing  in  the  Third  or  Last  Interglacial,  and 
being  found  in  even  greater  numbers  in  the  first  part  of  the  Fourth  or  Wisconsin 


HUMAN    EVOLUTION  •  337 

Glaciation.  Then,  quite  suddenly  (a  matter  of  centuries,  actually)  they  dis- 
appeared, being  replaced  throughout  their  range  by  men  like  ourselves.  The 
average  size  of  their  brains  (about  1450  cc)  was  somewhat  larger  than  the 
average  for  the  brain  of  living  men  (about  1350  cc).  The  skull  was  thick  walled 
and  low  and  bulged  at  the  sides,  with  the  rear  drawn  out  into  a  projecting 
occipital  region,  which  was  marked  by  a  ridge  for  the  attachment  of  massive 
neck  muscles.  The  retreating  forehead  sloped  back  from  heavy  brow  ridges,  and 
the  face  and  teeth  were  relatively  large.  The  lower  jaw  was  heavy,  but  lacked  the 
protruding  chin  of  modern  man.  The  rest  of  the  skeleton  indicates  that  Neander- 
thal men  were  only  about  five  feet  tall  but  of  an  exceptionally  powerful,  muscu- 


^^ti,:\s: 


ANCIENT  MEN 


Sterkfontein 


Fig.  33-6.  Fossil  hominid  skulls  of  the  Pleistocene  epoch.  Relative  age  is  shown 
by  position;  the  names  indicate  the  initial  place  of  discovery.  The  general  trends 
in  hominid  evolution  can  be  observed  from  the  Australopithecinae  at  the  bottom 
through  Pithecanthropus  (Java,  Peking,  and  Solo  men)  and  Neanderthal  (in- 
cluding Shanidar)  to  Homo  sapiens  (Cro-Magnon  and  Combe-Capelle)  at  the 
top.  The  Mount  Carmel  skull  shows  traits  of  both  Neanderthal  and  modern 
man.  (Redrawn  after  Washburn.) 


338  •  EVOLUTION   AND   MAN 

lar  build.  Because  of  these  rather  well-defined  differences  from  living  men,  the 
Neanderthals  have  been  placed  in  a  separate  species  of  the  genus  Homo,  H. 
neanderthalensis,  though  they  have  also  been  called  a  race  of  Homo  sapiens. 

Men  of  our  own  species,  Homo  sapiens,  do  not  appear  in  the  fossil  record 
until  about  35,000  B.C.  These  tall  and  well-built  men,  of  the  so-called  Cro- 
Magnon  type,  had  a  distinct  bony  chin  on  the  front  of  the  jaw,  a  high-domed, 
thin-walled  skull,  and  greatly  reduced  brow  ridges,  and  are  indistinguishable 
from  modern  men.  These  are  the  people  who,  in  a  relatively  short  time,  com- 
pletely replaced  the  Neanderthal  type.  However,  just  where  this  modern  type  of 
man  came  from  and  who  his  immediate  predecessors  were  are  far  from  clear.  At 
the  present  time  only  a  single  genus  of  the  family  Hominidae  and  a  single 
species  within  that  genus,  Homo  sapiens,  exists  on  the  earth.  All  mankind  be- 
longs to  this  one  species.  Human  fossils  of  types  clearly  belonging  to  the  genus 
Homo  have  been  found  as  far  back  as  the  late  Middle  Pleistocene,  but  the  record 
is  quite  fragmentary  and  incomplete,  and  the  relations  of  these  fragments  to  one 
another  and  to  modern  man  are  obscure.  See  Fig.  33-6. 

The  Origin  of  Modern  Man 

The  theories  of  the  origin  of  the  living  races  of  man  range  from  a 
simple  straight-line  evolution  from  Australopithecus  -»  Pithecanthropus  -» 
Neanderthal  — »  Modern  man,  to  a  polyphyletic  scheme  in  which  each  living 
human  race  is  derived  from  a  different  series  of  fossil  ancestors.  Although  it  is 
a  fairly  safe  assumption  that  neither  of  these  theories  is  correct,  the  available 
evidence  is  insufficient  to  establish  man's  lineage.  Many  of  the  known  fossil 
hominids  are  listed  in  Table  33-2  where  it  can  be  seen  that  fossils  of  rather 
different  types  (for  example,  Paranthropus  and  Pithecanthropus,  and  later, 
Pithecanthropus  and  Homo,  represented  by  the  Steinheim  and  Swanscombe 
skulls)  were  contemporary.  Such  information  suggests  that  in  the  Hominidae  as 
in  other  groups,  evolution  gave  rise  to  several  diverging  lines,  many  of  which 
became  extinct  while  others  eventually  gave  rise  to  new  species.  Though  only  one 
species,  Homo  sapiens,  now  exists,  it  has  already  diverged  to  some  extent  in  the 
formation  of  the  various  human  racial  groups.  The  details  of  the  skull  and  facial 
skeleton  of  Cro-Magnon  man  show  that  he  was  a  member  of  the  Caucasoid 
racial  group.  However,  it  does  not  necessarily  follow  that  the  other  races  are 
derived  from  the  white  race.  Indeed  it  is  unlikely  that  any  existing  race  was  an- 
cestral to  the  others.  Rather,  it  is  probable  that  all  of  the  living  races  have 
diverged  somewhat  from  the  ancestral  population  of  Homo  sapiens  from  which 
they  all  are  descended.  What  is  suggested  is  that,  even  though  the  exact  time  of 
origin  of  Homo  sapiens  is  not  yet  known,  the  Caucasoid  Cro-Magnon  men  show 
that  divergence  toward  modern  races  had  already  occurred  and  that  modern  man 
must  have  originated  at  some  time  prior  to  35,000  B.C. 


HUMAN    EVOLUTION  •   339 

The  relationship  between  Neanderthal  and  modern  men  has  constituted 
somewhat  of  a  puzzle.  Similarly,  while  it  is  reasonably  certain  that  the  modern 
Homo  sapiens  type  of  human  replaced  Neanderthal  man  throughout  his  range 
in  a  rather  brief  interval,  the  cause  of  his  extinction  remains  unknown.  Direct 
combat  leading  to  extermination  of  the  Neanderthals  may  be  the  answer,  but  it  is 
not  the  only  one  possible,  for  more  subtle  forms  of  competition — for  game  or 
caves,  for  example — could  have  had  the  same  ultimate  effect.  It  has  even  been 
suggested  that  where  the  two  groups  met,  they  interbred,  and  that  the  Neander- 
thals were  absorbed  rather  than  eliminated.  For  the  most  part,  the  evidence  does 
not  support  this  idea.  However,  on  the  eastern  shore  of  the  Mediterranean  in 
caves  on  the  slopes  of  Mount  Carmel  in  Palestine  have  been  found  skeletons 
that  show  a  strange  mixture  of  Neanderthal  and  sapiens  traits.  One  interpretation 
of  this  material  is  that  it  is  the  result  of  hybridization  between  the  two  groups. 
Although  it  is  true  that  the  Near  East  has  long  been  the  crossroads  of  the  world 
for  mankind,  and  from  that  standpoint  this  interpretation  seems  reasonable, 
nevertheless  other  explanations  have  also  been  advanced;  for  example,  that  these 
people  represent  the  last  stage  in  a  transition  from  Neanderthal  to  sapiens.  How- 
ever, other  evidence  makes  this  hypothesis  difficult  to  uphold.  For  one  thing, 
skulls  quite  different  from  Neanderthal  and  tending  toward  sapiens  are  already 
known  from  the  late  Middle  Pleistocene  (Steinheim  and  Swanscombe),  and 
definitely  sap  i  ens-like  skulls  (Fontechevade  and  Kan j era)  are  found  in  the  early 
Upper  Pleistocene,  well  before  the  time  of  the  Mount  Carmel  material  and  even 
before  the  time  of  the  Neanderthals  themselves.  Furthermore,  the  early  Neander- 
thal men  from  the  Third  Interglacial  were  not  as  extreme  in  their  distinguishing 
features  as  those  from  the  Fourth  Glacial.  What  this  evidence  suggests  is  that, 
rather  than  being  the  direct  ancestors  of  modern  man,  the  Neanderthalians  were 
a  divergent  group,  which  perhaps  became  especially  well  adapted  to  survive  the 
rigorous  climate  of  the  last  ice  age,  but  were  eventually  overrun  and  supplanted 
by  a  new  and  even  more  successful  human  type.  That  this  explanation  may  be 
correct  is  suggested  by  the  fact  that  the  new  people  apparently  brought  with 
them  a  new  and  more  advanced  culture.  The  Mousterian  tools  associated  with 
Neanderthal  man  were  replaced  by  the  more  refined  Aurignacian  stone  tools  of 
the  Upper  Paleolithic  men.  The  Neanderthalians  had  developed  a  distinctive  cul- 
ture of  their  own.  There  is  evidence  of  religious  concepts  in  their  ceremonial 
burial  of  the  dead  and  in  their  worship  of  cave  bears,  the  fearsome  enemies  with 
whom  they  fought  for  the  caves  essential  to  their  survival  during  the  last  ice  age. 
They  were  skilled  hunters,  able  to  take  game  as  large  as  the  mammoth  and  the 
woolly  rhinoceros.  However,  the  culture  of  their  Cro-Magnon  successors  was 
considerably  more  advanced,  marked  not  only  by  new  and  improved  stone  tools 
and  weapons,  but  by  evidence  of  great  hunting  skill  and  the  notably  graceful  art 
in  their  caves. 

At  present  we  have  a  glimpse  here  and  there  of  stages  in  human  evolu- 


340  •  EVOLUTION   AND   MAN 

tion  during  the  past  million  years  sufficient  to  show  that  evolution  in  the 
Hominidae  has  progressed  quite  rapidly  during  this  time.  The  major  adaptive 
shift  that  led  to  the  separation  of  the  hominids  from  the  apes  was  the  change  in 
the  lower  limbs  and  pelvis,  which  permitted  walking  erect.  This  shift  was  es- 
sentially complete  in  the  Australopithecinae,  and  the  evolution  of  a  progressively 
larger  brain  was  a  subsequent  development  Fossil  hominids  of  diverse  kinds  are 
widely  scattered  over  the  Old  World,  signs  of  a  successful,  expanding  group, 
but  the  place  of  origin  of  the  Hominidae  is  as  yet  unknown.  Although  indica- 
tions at  present  point  to  Africa,  this  may  be  simply  because  the  record  is  more 
complete  from  that  area.  Although  Pithecanthropus  (Java  and  Peking  men)  may 
be  regarded  as  a  stage  intermediate  between  the  Australopithecinae  and  modern 
man,  they  may  or  may  not  be  in  the  direct  line  of  descent.  Of  the  other  fossil 
men  available,  many  are  poorly  known,  either  because  only  a  few  fragments  have 
been  found  or  because  the  material  has  not  been  adequately  dated.  Although  the 
relationship  between  Neanderthal  man  and  modern  man,  as  represented  by  the 
Cro-Magnon  type  that  so  dramatically  superseded -the  Neanderthalians,  is  still  in 
doubt,  the  best  guess  is  that  both  were  derived  from  one  of  the  earlier  types  of 
Homo  now  known  only  from  a  few  scattered  skeletal  remains. 

Therefore,  the  fossil  record  of  man,  incomplete  and  fragmentary  though 
it  is,  is  sufficient  to  show  that  in  the  past  somewhat  different  human  types  did 
exist  from  which  modern  man  has  descended;  it  is  not  complete  enough  to  show 
exactly  what  the  course  of  evolution  leading  to  Homo  sapiens  has  been.  New 
human  fossils  are  being  found  at  an  accelerating  pace,  however,  and  there  is 
reason  to  hope  that  in  time  some  of  the  basic  questions  about  man's  origins  can 
be  answered  more  fully  than  at  present.  The  picture  may  appear  to  become  more 
confusing  before  it  is  clarified,  for  it  seems  unlikely,  since  isolation  exists  be- 
tween different  human  populations,  that  evolution  leading  to  man  would  follow 
a  simple,  straight-line  pattern  any  more  than  it  would  in  any  other  group. 

We  cannot  leave  our  discussion  of  man's  fossil  record  without  some 
mention  of  one  of  the  most  successful  hoaxes  in  history,  the  Piltdown  man, 
dignified  by  the  scientific  name,  Eoanthropus  dawsoni.  Fragments  were  reported 
from  a  gravel  pit  at  Piltdown  in  Sussex,  England,  between  1908  and  1915. 
When  reconstructed,  they  took  the  form  of  a  brain  case  much  like  that  of  modern 
man,  though  thicker,  and  a  lower  jaw  like  that  of  a  large  ape.  This  find  fulfilled 
the  then-current  concept  of  what  the  "missing  link"  between  man  and  the  apes 
would  be  like.  Accepted  as  authentic,  studied  and  puzzled  over  by  experts,  the 
Piltdown  man  went  unexposed  for  over  forty  years.  The  subsequent  finds  of 
fossil  hominids,  especially  the  australopithecines,  made  an  ever-greater  anomaly 
of  Eoanthropus,  for  they  all  agreed  in  having  hominid  jaws  and  dentition  asso- 
ciated with  a  skull  rather  like  an  ape's  instead  of  the  reverse.  Eventually  with  the 
aid  of  modern  techniques,  the  Piltdown  man  was  shown  beyond  question  to  be 
a   clever   fraud   concocted    from   a   human   skull   and   the   carefully   doctored 


HUMAN    EVOLUTION  •  34 1 

jaw  and  teeth  of  an  orangutan.  Even  the  tools  and  animal  fossil  bones  found  at 
the  same  site  turned  out  to  have  been  planted.  Surely  this  was  one  of  the  most 
successful  practical  jokes  in  history,  but  modern  methods  of  dating  and  analysis, 
if  not  the  lesson  learned  here,  make  it  very  improbable  that  anthropologists  will 
ever  again  be  fooled  in  this  way. 

Man,  a  Polytypic  Species 

Although  it  has  been  argued  that  there  are  several  living  human  species, 
it  is  clear  that  if  the  same  taxonomic  criteria  are  applied  to  man  as  have  been 
applied  to  other  species,  there  is  but  one  human  species  living  at  the  present 
time.  This  species,  Homo  sapiens,  is  polymorphic,  for  every  human  population 
manifests  considerable  variability,  a  fact  easily  confirmed  by  a  quick  glance  at 
your  friends  and  neighbors.  It  is  also  polytypic,  for  many  geographic  subspecies 
have  been  distinguished  and  named.  They  are  not  separate  species,  however, 
because  the  different  races  can  and  do  interbreed.  Probably  the  only  racial  cross 
that  has  not  occurred  is  between  Eskimos  and  African  Bushmen.  Furthermore,  it 
is  not  possible  to  draw  sharp,  distinct  lines  of  demarcation  between  human  racial 
groups  since  one  race  usually  blends  into  another  in  the  zone  of  contact.  The 
different  human  races  differ  from  each  other  in  the  incidence  of  certain  of  their 
genes,  and  this  is  the  basic  distinction  between  races.  While  all  living  men  must 
share  fundamentally  similar  genotypes  that  cause  them  to  develop  into  members 
of  Homo  sapiens,  different  human  populations  have  diverged  from  one  another 
to  some  extent.  Human  populations,  past  and  present,  are  subject  to  the  effects 
of  mutation,  natural  selection,  random  genetic  drift,  and  gene  flow  just  as  are 
other  species. 

Far  too  little  is  known  about  adaptive  values  in  man,  and  man's  present 
high  mobility  tends  to  obscure  still  further  his  adaptations  to  local  conditions, 
but  the  indications  are  that  the  different  human  races  are  adapted  to  their  imme- 
diate environments.  The  relation  between  degree  of  skin  pigmentation  and 
amount  of  exposure  to  the  sun  is  a  familiar  example,  but  perhaps  a  somewhat 
shaky  one  since  the  skin  has  functions  other  than  to  serve  as  a  filter  for  the  ultra- 
violet light  needed  to  form  vitamin  D  in  the  body.  Body  form  shows  an  even 
closer  relation  to  climate  than  does  skin  color.  The  surface-to-volume  ratio  is 
maximized  for  more  efficient  heat  dissipation  in  the  lanky  desert  Arabs  and 
Nilotic  Negroes  living  under  the  searing  tropical  sun,  but  is  minimized  in  the 
roly-poly  Eskimos.  The  nasal  cavities  of  Eskimos  and  north  Europeans  have  also 
been  shown  to  be  better  suited  for  warming  and  moistening  cold,  dry  air  than 
those  of  peoples  living  under  milder  climates.  In  fact,  the  entire  Mongoloid  face 
is  thought  to  be  adapted  for  life  in  a  cold  climate,  for  the  nose  is  reduced  and 
the  entire  face  is  flattened  out  and  padded  with  fat,  and  the  eyes  are  protected  by 
the  so-called  Mongoloid  fold.  The  steatopygia  or  fat  on  the  buttocks  of  African 


342  •  EVOLUTION   AND   MAN 

Bushwomen  is  another  trait  often  cited  as  adaptive,  for  they  store  fat  there  in 
remarkable  quantities.  Although  it  has  been  suggested  that  steatopygia  is  func- 
tionally analagous  to  a  camel's  hump — an  energy  reserve  that  does  not  limit  heat 
dissipation — this  explanation  fails  to  explain  why  the  trait  is  absent  in  the  male. 
It  may  be  related  to  food  storage  for  sustaining  pregnancy,  but  sexual  selection 
may  also  play  a  role,  for  the  trait  is  said  to  be  much  admired  by  the  men. 

The  sickle  cell  gene  discussed  earlier  is  one  of  the  best-understood  cases 
of  adaptation  in  man.  In  regions  where  malaria  is  prevalent,  the  heterozygote  for 
this  gene  is  better  adapted  to  survive  than  either  homozygote;  for  one  individual 
(Hbs/Hbs)  is  done  in  by  his  harmful  genes,  whereas  the  other  (Hba/Hba)  is 
apt  to  be  carried  off  by  malaria.  Hence  a  balanced  polymorphism  due  to  heterosis 
exists,  and  in  some  regions  over  40  percent  of  the  population  may  carry  the  sickle 
cell  gene,  a  high  frequency  out  of  all  proportion  to  what  might  be  expected  of  a 
gene  with  such  drastic  effects  in  the  homozygous  condition.  Though  this  gene  is 
most  common  in  Negro  Africa  where  its  highest  frequencies  coincide  roughly 
with  the  highest  incidence  of  malaria,  it  is  not  restricted  to  this  region  or  to  this 
race,  for  it  has  also  been  found  in  malarial  regions  of  India,  Greece,  Italy, 
Turkey,  and  Arabia.  The  most  reasonable  explanation  for  the  distribution  of  the 
sickle  cell  gene  is  that  it  arose  by  mutation,  probably  among  the  Negroes  in 
Africa,  and  has  been  introduced  into  other  regions  and  races  by  gene  flow 
through  occasional  matings  between  the  Mediterranean  peoples  and  Negro  car- 
riers. Once  established,  its  frequency  increased  owing  to  its  selective  advantage 
in  malarial  areas.  It  is  not,  however,  found  in  all  regions  of  the  world  where 
malaria  exists,  presumably  because  it  never  got  there  either  by  mutation  or  by 
migration.  However,  other  genes  similar  in  function  but  distinct  from  the  sick- 
ling gene  have  been  discovered.  As  a  final  footnote  to  this  story,  the  primary 
effect  of  the  sickle  cell  gene,  so  far-reaching  in  its  ultimate  effects,  has  been 
shown  to  be  merely  the  substitution  in  normal  adult  hemoglobin  of  a  single 
amino  acid,  valine,  for  another,  glutamic  acid,  in  one  of  the  peptides  making  up 
the  hemoglobin  protein  molecule. 

Many  questions  remain  to  be  answered.  What  adaptive  value,  if  any,  is 
there  in  the  different  eye  colors  in  man  or  in  the  different  color  and  shape  of 
human  hair?  Why  do  some  races  have  much  more  body  hair  than  others?  What 
factors  are  responsible  for  the  development  of  the  pygmy  tribes  ?  The  list  could 
be  considerably  extended,  but  the  answers  in  nearly  all  cases,  are  unknown  or  at 
best  merely  informed  guesses.  In  principle,  we  know  that  the  differences  must 
have  arisen  through  the  combination  of  directive  and  chance  elements  that  govern 
the  course  of  evolution  within  breeding  populations  (mutation,  selection,  genetic 
drift,  and  migration) ;  in  detail,  however,  our  knowledge  of  the  origin  and 
function  of  the  traits  that  distinguish  one  human  race  from  another  is  quite 
sketchy.  Many  traits  seem  unlikely  to  confer  any  adaptive  value,  but  even  this 
assumption  cannot  be  taken  for  granted.  The  different  blood  groups  of  the  ABO 


HUMAN    EVOLUTION  •  343 

system  were  long  cited  as  traits  in  man  governed  by  neutral  genes,  but  it  now 
appears,  for  example,  that  stomach  cancer  is  somewhat  more  likely  to  develop  in 
people  of  type  A,  and  people  of  type  O  are  somewhat  more  susceptible  to  duo- 
denal ulcers,  and  thus  this  example  must  be  discarded. 

The  Races  of  Man 

There  is  not  and  probably  cannot  be  any  general  agreement  on  the 
number  of  distinct  human  races.  More  than  thirty  have  been  distinguished. 
However,  at  least  six  rather  distinct  racial  groups  can  be  recognized  as  follows 
(see  also  Fig.  33-7)  : 

Race  Distribution  before  1492 

1.  Negroid  Widely  scattered.  Tropical  Africa  and  Old  World  tropics — 

India,    Andaman   Islands,    Philippines,   Queensland,   New 
Guinea,  islands  east  to  Fiji  and  southeast  to  New  Caledonia 

2.  Caucasoid  North  of  tropics  in  North  Africa,  Europe,  and  Western 

Asia,  southeast  into  tropics  in  India 

3.  Mongoloid  North  and  East  Asia,  south  into  Sunda  Islands,  North  and 

South  America 

4.  Bushmen  South  Africa 

5.  Australoid  Australia 

6.  Polynesian  Remote  Central  Pacific  islands  from  New  Zealand  to  Hawaii 

In  terms  of  numbers  and  widespread  distribution,  the  Negroid,  Cau- 
casoid, and  Mongoloid  groups  are  the  three  major  human  races  at  the  present 
time.  Negroids  are  usually  dark  skinned  with  black  woolly  hair,  broad,  flat  noses, 
and  thick  lips.  Caucasoids  generally  have  rather  light  skin,  long,  narrow  noses, 
and  relatively  straight  hair.  The  hair  of  Mongoloids  is  straight  and  black,  an  eye 
fold  is  common,  and  the  face  is  flattened  with  high  cheekbones.  As  soon  as  these 
descriptions  have  been  given,  they  must  immediately  be  qualified  because  there 
is  a  great  deal  of  variation  within  each  group.  All  the  races  vary  considerably  in 
skin  color,  for  example.  The  Caucasoid  or  so-called  "white"  race  varies  all  the 
way  from  the  blond,  blue-eyed  Scandinavian  to  the  dark-eyed,  dark-skinned 
Hindu  of  India.  The  Mongoloid  group  includes  not  only  the  "yellow"  skinned 
Asians  and  Eskimos  but  the  American  "redskin."  In  size,  Negroids  vary  from 
the  tall  Watussi  (Batutsi)  tribe  whose  members  approach  seven  feet,  to  the 
Pygmies  whose  males  average  under  five  feet  in  height.  Furthermore,  the  con- 
tacts between  Negroids  and  Caucasoids  in  northern  Africa  and  between  Mongo- 
loids and  both  Negroids  and  Caucasoids  in  the  Orient  have  effectively  blurred 
any  distinctions  between  the  races.  In  fact,  the  concept  of  "pure  races,"  the  idea 
that  Homo  sapiens  in  prehistoric  times  consisted  of  a  group  of  separate,  distinct 


344  •  EVOLUTION   AND   MAN 

racial  groups  whose  differences  are  gradually  being  eroded  away  by  the  coming 
of  civilization  is  so  improbable  as  to  be  relegated  to  the  realm  of  myths.  Al- 
though local  populations  in  the  past  undoubtedly  were  somewhat  more  isolated 
than  at  present,  variability  within  populations  and  gene  flow  between  popula- 
tions, then  as  now,  would  have  prevented  the  development  of  a  "pure  race." 


f":'S."  &£? 


Fig.  33-7.  Representatives  of  major  human  races,  (a)  Mongoloid:  Alaskan 
Eskimo  woman;  {b)  Negroid:  South  African  Bantu  woman;  (c)  Bushmen: 
Hottentot  woman  with  steatopygia;  (d)  Australoid:  Girl  from  Northern  Aus- 
tralia; (e)  Polynesian:  Maori  woman;  (/)  Caucasoid:  United  States.  (Courtesy 
of  Peabody  Museum,  Harvard  University.) 


HUMAN   EVOLUTION  •  345 

The  Nazi  concept  of  a  pure  Nordic  race  as  the  original  Europeans  and  the 
builders  of  modern  civilization  simply  does  not  stand  up  in  the  light  of  our 
knowledge  of  modern  genetics  and  anthropology.  Human  populations  have 
never  been  static  entities.  They  have  adapted  to  changing  physical  and  biological 
conditions.  The  net  result  of  natural  selection,  hybridization,  mutation,  and 
genetic  drift  has  been  an  ever-shifting  pattern  in  human  breeding  populations. 
Some  have  disappeared,  either  completely  or  by  absorption  into  others  by  inter- 
marriage, while  distinctive  new  populations  have  appeared.  In  the  past,  isolation 
by  distance  appears  to  have  been  the  significant  factor  that  permitted  the  differ- 
entiation of  Homo  sapiens  into  recognizably  different  racial  groups.  At  present, 
isolation  is  breaking  down,  and  new  and  different  human  types  are  arising  as  the 
result  of  hybridization.  The  mestizos  of  Latin  America,  a  mixture  of  European, 
Indian,  and  some  Negro  ancestry,  and  the  inhabitants  of  Pitcairn  Island,  de- 
scended from  Europeans  and  Polynesians,  represent  examples  of  this  sort. 

The  remaining  three  races  of  man  mentioned  above,  the  Polynesians, 
the  Australoids,  and  the  African  Bushmen  and  their  Hottentot  relatives,  are  in  a 
sense  peripheral  human  groups.  The  Bushmen  are  found  only  in  southern  Africa, 
the  Australoids  in  the  Australian  region,  and  the  Polynesians  on  the  islands  in 
the  far  reaches  of  the  Pacific.  This  distribution  pattern  calls  to  mind  the  distribu- 
tion of  relict  populations  in  our  discussion  of  biogeography.  Although  the 
analogy  may  hold  with  respect  to  the  Bushmen  and  the  Australoids,  who  appear 
to  have  occupied  their  present  territory  for  some  time,  the  Polynesians  seem  to 
have  reached  their  island  realm  only  quite  recently.  The  Polynesians,  despite  the 
arguments  based  on  the  Kon-Tiki  voyage,  appear  very  definitely  to  have  originated 
in  Asia  and  not  in  South  America. 

Cultural  Evolution 

In  addition  to  his  own  fossil  remains,  early  man  left  behind  him  another 
type  of  record,  a  record  of  his  culture.  These  cultures  are  known  as  the  Paleo- 
lithic (or  Old  Stone  Age) ,  Mesolithic,  Neolithic,  and  the  Bronze  and  Iron  Ages. 
These  broad  stages  are  used  to  indicate  the  cultural  status  achieved  by  a  people 
and  do  not  necessarily  indicate  absolute  divisions  of  time,  for  some  peoples  are 
just  now  emerging  from  the  Neolithic,  a  stage  through  which  others  passed 
several  thousand  years  ago.  The  nature  of  past  human  cultures  is  inferred  from 
the  form  of  their  tools  and  weapons  and  other  implements  (see  Fig.  33-8).  The 
Stone  Age  ranged  in  time  from  the  Pliocene  up  until  a  few  thousand  years  ago. 
The  Neolithic,  which  marked  the  invention  of  agriculture,  began  only  about 
ten  thousand  years  ago;  man,  therefore,  has  been  a  hunter  and  gatherer  of  wild 
plant  food  for  all  but  about  1  percent  of  his  known  existence.  Even  a  high  level 
of  skill  in  hunting  was  reached  only  about  thirty-five  or  forty  thousand  years  ago 
in  the  Upper  Paleolithic.  Hence  early  cultures  changed  only  very  slowly  and 


346  •  EVOLUTION   AND   MAN 


UPPER  PALEOLITHIC 


\  V 


<35^ 


BLADE  TOOLS 


MOUSTERIAN 


S\  .4 


111 


FLAKE  TOOLS 


ACHEULEAN  (MIDDLE) 


ABBEVILLEAN 

CORE  TOOLS 


PEBBLE  TOOLS 


Fig.  33-8.  The  tool  traditions  of  Europe  form  the  basis  for  classifying 
Paleolithic  cultures.  The  tools  are  arranged  according  to  age,  with  the 
oldest  at  the  bottom.  Two  views  of  each  tool  are  given  except  for  the 
blade  tools,  which  are  shown  in  three  views.  Tool  traditions  have  been 
named  for  the  site  of  discovery.    (With  permission  of  Washburn.) 


HUMAN   EVOLUTION  •  347 

persisted  for  long  periods,  but  the  pace  of  cultural  changes  has  been  ever 
increasing. 

The  stone  implements  were  made  by  chipping  and  flaking  pieces  from 
a  flint  core  to  fashion  the  desired  tool  or  weapon,  and  in  some  cultures  the  flakes 
were  also  used  for  a  variety  of  smaller  implements.  Since  the  tools  were  fash- 
ioned with  different  techniques  and  varying  degrees  of  skill  and  complexity,  it 
has  been  possible  to  recognize  a  number  of  different  tool  traditions,  and  these 
have  usually  been  named  after  the  place  where  they  were  first  discovered.  Since 
tool  making  underwent  gradual  change  and  improvement,  the  evolution  concept 
has  been  applied  to  the  succession  of  tool  traditions.  Although  perhaps  useful 
for  descriptive  purposes,  such  application  holds  certain  pitfalls,  for  the  evolution 
of  tools  is  not  biological  evolution  and  does  not  necessarily  parallel  the  biological 
evolution  that  must  have  been  going  on  in  man  at  the  same  time.  Furthermore, 
two  quite  different  types  of  men  could  learn  to  fashion  the  same  type  of  tool. 
Hence,  a  new  type  of  inheritance,  cultural  inheritance,  appears.  Cultural  patterns 
and  traditions  could  not  only  be  passed  from  one  generation  to  its  successors,  but 
could  be  imitated  and  widely  and  rapidly  disseminated  without  the  necessity  for 
any  sort  of  biological  continuity.  Thus  the  attempts  to  link  a  particular  tool  tradi- 
tion with  a  particular  kind  of  fossil  man  are  really  valid  only  when  there  is  posi- 
tive evidence  of  association. 

The  earliest  recognizable  tools  were  associated  with  the  Villafranchian 
fauna.  The  recent  discovery  of  Zinjanthropus  in  association  with  stone  tools 
of  the  pre-hand-axe  Oldowan  type  shows  that  even  the  australopithecines  had  a 
true  stone  culture.  The  Abbevillian  and  Chellean  hand-axe  cultures  were  suc- 
ceeded by  the  advanced  Acheulean  type  of  hand  axe.  The  Clactonian  flake  in- 
dustry, contemporaneous  with  these  early  hand-axe  based  cultures,  was  followed 
by  the  Mousterian-Levalloisian  type  of  stone  implements,  which  were  more  elab- 
orate and  carefully  made  than  anything  that  preceded  them.  The  Mousterian 
stone  tools  seem  to  have  been  fashioned  by  Neanderthal  men,  for  Neanderthal 
skeletal  remains  have  often  been  found  with  Mousterian  weapons  and  tools.  The 
rapid  replacement  of  H.  neanderthalensis  by  H.  sapiens  in  Europe  coincided  with 
the  appearance  of  Aurignacian  implements,  followed  in  a  relatively  short  period 
by  the  Solutrean  and  Magdalenian  types.  It  seems  safe  to  assume  that  this  new 
kind  of  man  had  developed  new  capabilities  in  fashioning  his  tools  and  weapons, 
for  they  were  of  a  refinement  and  variety  not  previously  seen.  In  addition  to 
stone  he  used  materials  such  as  bone,  horn,  and  ivory  to  fashion  ornaments  as 
well  as  weapons  and  tools.  The  Paleolithic,  then,  endured  for  by  far  the  greater 
part  of  man's  existence  and  was  marked  by  gradual  but  accelerating  advances  in 
his  ability  to  fashion  stones  and  other  materials  to  his  own  uses.  The  conclusion 
is  difficult  to  avoid  that  the  advances  were  so  slow  at  first  because  the  earlier 
species  of  men  were  of  a  lower  order  of  intelligence  than  the  men  who  followed 
them. 


348  •  EVOLUTION   AND   MAN 

With  the  passing  of  the  last  ice  age  about  10,000  years  ago  a  new 
phase  of  culture,  the  Mesolithic,  appeared.  These  people  both  hunted  and  fished, 
for  not  only  did  they  make  bows  and  arrows,  but  they  fashioned  nets  and  canoes 
and  lived  on  fish  and  shellfish  as  well  as  on  game. 

The  Neolithic  is  marked  by  the  appearance  of  ground  and  polished 
stone  tools,  and  by  pottery  and  weaving,  but  the  real  significance  of  the  New 
Stone  Age  lay  in  the  invention  of  agriculture.  It  was  the  domestication  of  plants 
and  animals  that  permitted  man  to  give  up  his  essentially  nomadic  existence  and 
to  settle  down  in  relatively  permanent  communities.  Agriculture  can  support 
greater  numbers  of  people  than  a  hunting  and  gathering  culture.  Only  with  this 
advance  did  modern  civilization  become  possible.  The  oldest  known  ground 
stone  tools,  cultivated  plants,  and  domesticated  animals  (except  for  the  dog) 
come  from  southwestern  Asia  and  are  less  than  ten  thousand  years  old.  This 
period  seems  even  shorter  when  it  is  realized  that  agriculture  was  invented  less 
than  400  generations  ago.  Agriculture  apparently  arose  independently  in  at  least 
three  separate  places.  In  southwestern  Asia  it  was  based  on  wheat,  in  south- 
eastern Asia  on  rice,  and  in  the  Americas  on  maize.  The  stone  implements  of  the 
Neolithic  were  soon  augmented  by  implements  made  of  new  materials,  and  the 
Bronze  Age,  which  spread  from  the  Near  East,  was  soon  followed  by  the  Iron 
Age.  These  early  civilizations  bring  us  up  to  the  beginnings  of  recorded  history. 

With  the  development  of  civilization  and  culture,  man  has  become  a 
biologically  dominant  species  that  has  expanded  its  range  to  the  farthest  corners 
of  the  earth  and  greatly  increased  in  numbers.  He  is  now  cosmopolitan,  the 
dominant  mammalian  species  in  all  parts  of  the  world,  who  has  no  reason  to 
fear  any  competing  species  or  predators,  so  complete  is  his  domination  by  means 
of  his  weapons.  Furthermore,  he  has  gained  mastery  over  most  of  his  parasites 
and  has  remodeled  his  environment,  using  other  species  for  his  purposes.  All  of 
these  developments  became  possible  with  the  evolution  of  the  human  brain,  the 
source  of  man's  adaptive  advantage  over  all  other  species.  Human  evolution  has 
reached  a  new  plateau,  for  superimposed  on  the  biological  evolution  that  still 
continues  in  man  is  cultural  evolution.  This  new  facet  in  evolution,  the  trans- 
mission of  knowledge  through  culture,  has  opened  up  new  vistas.  Not  only  has 
he  controlled  the  evolution  of  other  species  as  he  has  modified  domesticated 
plants  and  animals  better  to  serve  his  needs,  but  he  now  has  sufficient  knowledge 
to  control  the  course  of  his  own  evolution.  Human  cultural  and  biological  evolu- 
tion are  going  to  continue  in  any  event.  The  fundamental  question  is  whether 
man  has  the  wisdom  to  guide  his  own  future. 


SUGGESTED  READING 

Boule,  M.,  and  H.  V.  Vallois,  1957.  Fossil  men.  London:  Thames  and  Hudson. 
Boyd,  W.  C,  1950.  Genetics  and  the  races  of  man.  Boston:  Little,  Brown. 


HUMAN  EVOLUTION    •  349 

Clark,  W.  E.  L.,  1955.  The  fossil  evidence  for  human  evolution.  Chicago:  University 

of  Chicago  Press. 
,    1957.    History   of   the   primates.   Chicago:    University   of   Chicago   Press, 

Phoenix  Books. 
,  1959.  "The  crucial  evidence  for  human  evolution,"  Proc.  Amer.  Philosoph- 
ical Society,  703(2)  :159-172. 

,  1959.  The  antecedents  of  man.  Edinburgh:  Edinburgh  University  Press. 

Coon,  C.  S.,  1954.  The  story  of  man.  New  York:  Knopf. 

Dart,  R.  A.,  and  D.  Craig,  1959.  Adventures  with  the  missing  link.  New  York: 

Harper. 
Demerec,  M.,  ed.,  1950.  "Origin  and  evolution  of  man,"  Cold  Spring  Harbor  Symp. 

Quant.  Biol.,  15.  New  York:  Long  Island  Biological  Assoc. 
Evolution   and   anthropology:   a   centennial  appraisal,    1959.   Washington,   D.   C: 

Anthropol.  Society. 
Howells,  W.,  1959.  Mankind  in  the  making.  Garden  City,  N.  Y.:  Doubleday. 
Kluckhohn,  C,   1949.  Mirror  for  man.  New  York:  McGraw-Hill.   (Also  Premier 

Reprint,  1957.) 
Tax,  S.,  ed.,  I960.  Evolution  after  Darwin,  Vol.  2,  The  evolution  of  man.  Chicago: 

University  of  Chicago  Press. 
"The  human  species,"  Scientific  American,  203(3)  Sept.  I960. 
Weiner,  J.  S.,  1955.  The  Piltdown  forgery.  New  York:  Oxford  University  Press. 


CHAPTER 


34 

Radiation,  Genetics,  and  Man 


One  has  only  to  look  at  his  friends  and  relations  to  get 
some  idea  of  the  variation  that  exists  in  a  natural  population. 
Some  of  this  variation,  of  course,  is  of  environmental  origin.  The 
genetic  portion  is  due  either  to  the  recombination  and  interaction 
of  existing  genes  or  to  new  mutations.  Since  existing  genes  at 
some  time  in  the  past  also  arose  through  mutation,  mutation 
looms  large  as  a  source  of  variation.  Mutations  have  been  denned 
as  self-duplicating  changes  in  the  hereditary  material.  In  a  broad 
sense,  they  include  submicroscopic  point  mutations  and  the  micro- 
scopically detectable  rearrangements  following  chromosome  break- 
age. The  "spontaneous"  mutations  may  be  due  to  the  natural  or 
background  radiation  coming  from  radioactive  minerals  and  cos- 
mic rays.  Background  radiation  alone  is  insufficient  to  account  for 
all  "spontaneous"  mutations,  but  a  variety  of  chemical  mutagens 
has  been  discovered,  and  these  plus  the  effects  of  temperature  and 
the  mutation  rate  genes  mentioned  previously  undoubtedly  play 
a  role  in  the  induction  of  naturally  occurring  mutations. 


The  Frequency  of  Harmful  Genes 

The  great  majority  of  "spontaneous"  point  mutations  are 
deleterious.  It  has  been  estimated  that  at  a  maximum  only  1  in 
1000  is  beneficial  under  existing  conditions.  The  reasons  for  this 
fact  are  fairly  simple.  Existing  genes  are  the  product  of  prior 
evolution  and,  since  they  have  survived  the  winnowing  action  of 
natural  selection,  they  give  rise  to  well-adapted  organisms.  Hence, 


350 


RADIATION,    GENETICS,    AND    MAN  •   351 

any  change  in  an  existing  gene  is  far  more  likely  to  impair  its  function 
than  to  improve  it.  Most  genes  appear  to  be  concerned  with  the  presence 
and  specificity  of  enzymes,  and  mutations,  in  disrupting  the  metabolic  pat- 
tern, are  generally  harmful.  For  every  lethal  mutation,  it  is  estimated 
that  four  detrimental  mutations,  reducing  viability  at  least  10  percent,  occur. 
Since  this  estimate  is  based  on  radiation-induced  mutations  in  Drosophila 
(Fig.  34-1),  the  proportion  of  detrimentals  among  spontaneous  mutants  may 
actually  be  higher  than  four  to  one. 

Most  new  mutations  are  recessive.  In  other  words,  the  normal  gene  is 
effective  in  a  single  dose  in  masking  or  covering  up  the  effects  of  the  deleterious 
or  lethal  mutant.  Less  than   1  in   100  mutants  is  fully  dominant.  Therefore, 


DOSE  IN  rx  10* 


Fig.  34-1.    Linear   relation   between   radiation   dose   and   mutation 
rate   for   sex-linked   lethals   in   Drosophila.     (With   permission   of 

Begg.) 


352  •  EVOLUTION   AND   MAN 

contrary  to  a  widespread  belief,  mutation  does  not  lead  at  once  to  a  host  of 
monsters  in  the  next  generation.  The  rare  dominants  are  rapidly  eliminated  by 
natural  selection,  dominant  lethals  disappearing  in  the  first  generation.  The  re- 
cessives  are  added  to  the  gene  pool  of  the  population.  They  will  produce  maxi- 
mum damage  only  when  present  in  double  dose,  which  may  not  occur  for  many 
generations.  However,  the  recessive  mutants  are  generally  not  completely  reces- 
sive, for  two  doses  of  the  normal  gene  ordinarily  are  better  than  a  single  dose 
plus  the  mutant  and  hence  harmful  mutations  can  cause  damage  even  when 
heterozygous.  This  damage  may  be  very  difficult  to  detect  since  it  is  on  the  order 
of  a  2  to  4  percent  reduction  in  fecundity,  fertility,  viability,  or  longevity,  with 
no  obvious  visible  defects.  Thus,  a  gene  mildly  deleterious  in  single  dose  may 
eventually  do  as  much  harm  as  a  grossly  harmful  one,  for  it  persists  longer  and 
has  a  chance  to  cause  impairment  to  more  individuals.  Eventually  it  will  lead  to 
the  extinction  or  "genetic  death"  of  the  line  of  descent  carrying  it,  and  this  will 
usually  happen  before  it  becomes  homozygous. 

At  the  present  time  4  to  5  percent  of  the  children  born  alive  in  the 
United  States  are  in  some  way  defective.  This  startling  statistic  may  at  first  glance 
seem  unreasonably  high,  but  it  includes  not  only  congenital  malformations  but 
mental  deficiency  and  epilepsy,  and  defects  of  vision  or  hearing  and  of  the 
gastrointestinal,  genitourinary,  neuromuscular,  hematological,  and  endocrine  sys- 
tems. When  it  is  realized  that  estimates  of  the  frequency  of  mental  deficiency 
alone  range  as  high  as  5  percent  in  this  country,  the  above  estimate  seems  fairly 
conservative.  About  half  of  these  children,  or  2  percent  of  the  total  live  births, 
are  suffering  from  disorders  that  have  a  simple  genetic  origin  and  will  appear 
prior  to  sexual  maturity.  Thus,  of  the  next  100  million  children  born  in  the 
United  States,  two  million  can  be  expected  to  have  some  sort  of  hereditary 
defect.  These  defects  are  the  result  of  deleterious  "spontaneous"  mutants  induced 
in  the  past  by  natural  causes  and  now  present  in  the  gene  pool  of  our  population. 

Many  of  these  inherited  conditions  are  severe  enough  to  cause  the  death 
of  the  child  or  else  to  limit  or  prevent  his  reproduction.  These  defective  genes, 
then,  are  constantly  being  eliminated  from  the  population  by  natural  selection. 
Why,  if  these  genes  have  been  selected  against  for  centuries,  are  they  still  so  fre- 
quent? The  answer  is  that  they  are  being  generated  by  recurrent  spontaneous 
mutations.  An  equilibrium  between  their  rate  of  origin  by  mutation  and  their 
rate  of  elimination  by  selection  has  been  approximated. 

In  this  connection  it  may  be  pointed  out  that  the  practice  of  medicine 
has  changed  radically  in  the  past  100  years.  One  hundred  years  ago  the  major 
killers  of  human  beings  were  infectious  diseases.  Today,  where  modern  medicine 
is  practiced,  the  physician  is  turning  his  attention  away  from  combating  micro- 
organisms. (The  microorganisms  have  by  no  means  surrendered;  the  origin  of 
resistant  strains  has  tempered  the  initial  optimism  that  greeted  the  various  anti- 
biotics and  chemotherapeutic  agents.)  The  major  causes  of  death  at  the  present 


RADIATION,   GENETICS,   AND    MAN  •  353 

time  do  not  involve  infectious  organisms,  but  they  do  involve,  to  varying  degrees, 
harmful  genes,  the  new  objects  of  medical  assault.  To  the  extent  that  the  physi- 
cian succeeds  in  combating  the  effects  of  deleterious  genes  by  suitable  environ- 
mental manipulations  (for  example,  insulin  for  diabetes),  the  results  are 
dysgenic,  for  the  proportion  of  these  genes  in  the  population  will  increase  in  subse- 
quent generations.  The  physicians  of  the  next  generation,  therefore,  will  have  a 
greater  proportion  of  such  cases  to  treat.  It  is  estimated  that  the  average  person 
carries  the  equivalent  of  about  4  genes,  any  one  of  which,  in  the  homozygous 
condition,  would  cause  his  death.  In  other  words,  he  may  carry  4  lethals,  or 
8  deleterious  genes  with  a  50  percent  probability  of  causing  death,  or  100  genes 
with  only  a  4  percent  chance.  Actually,  there  is  undoubtedly  a  mixture  of  these 
types  descended  from  past  mutations  that  still  persist  in  the  population.  Unless 
some  way  is  found  to  prevent  their  increase  in  frequency,  this  load  of  hidden 
mutations  will  inevitably  become  heavier  as  the  medical  control  of  genetic  defects 
improves. 

The  average  spontaneous  mutation  rate  for  a  given  gene  locus  has  been 
estimated  to  be  from  1  to  2  new  mutations  per  100,000  genes  per  generation. 
This  statement  means  that  in  100,000  sperm  cells,  1  or  2  can  be  expected  to 
carry  a  newly  arisen  spontaneous  mutation  for  a  particular  gene.  However,  the 
total  rate,  a  measure  of  the  mutations  at  all  gene  loci  on  all  chromosomes,  is  con- 
siderably higher.  The  total  number  of  genes  is  not  known  in  any  species.  Indirect 
estimates  lead  to  a  value  of  at  least  10,000  gene  loci  in  Drosophila,  and  this 
figure  is  probably  a  conservative  estimate  for  man.  The  total  mutation  rate  there- 
fore equals  ■  X  10,000  = .  Hence,  1  in  10  gametes  or  about  2  in 

^  100,000  10  6 

10  diploid  individuals  can  be  expected  to  carry  a  newly  arisen  mutation.  At 
equilibrium,  this  frequency  represents  the  risk  of  genetic  death  since  the  rate  of 
elimination  of  the  mutants  equals  their  rate  of  origin.  This  risk  is  shared  by  all 
of  us  since  everyone  carries  several  to  many  detrimental  mutations.  It  indicates 
the  loss  of  fitness  of  the  average  individual  as  compared  to  a  hypothetical  person 
with  no  detrimental  mutants  at  all.  It  should  be  pointed  out  that  the  above  esti- 
mates are  based  primarily  on  data  from  Drosophila  and  mice,  with  the  mutation 
rates  in  mice  somewhat  higher  than  those  of  the  fruit  flies.  Man's  mutation  rate, 
because  of  his  greater  generation  length,  is  apt  to  be  higher  than  that  of  mice. 
More  and  better  data  for  man  are  needed,  but  the  fundamental  conclusions  are 
unlikely  to  change. 


Genetic  Effects  of  Radiation 

The  original  discovery  of  the  mutagenic  effects  of  x-rays  by  Muller  in 
1927  was  of  great  interest  to  geneticists,  but  only  with  the  coming  of  the  atomic 
age  have  the  biological  effects  of  ionizing  radiations  become  of  general  concern. 


354  •  EVOLUTION   AND   MAN 

The  biological  effects  are  of  two  major  kinds:  somatic  or  direct  effects  on  ex- 
posed individuals,  causing  either  death  or  immediate  or  delayed  pathological 
effects;  and  genetic  effects  in  the  germ  cells  of  exposed  individuals,  which  are 
then  transmitted  to  subsequent  generations.  Ionizing  radiations  such  as  x-rays 
and  gamma  rays  (similar  to  x-rays  but  emanating  from  radioactive  substances 
such  as  radium)  have  been  shown  to  induce  chromosome  breakage  as  well  as 
gene  or  point  mutations.  In  their  passage,  these  radiations  break  existing  chem- 
ical bonds  and  lead  to  the  formation  of  positively  and  negatively  charged  ions. 
Presumably  the  biological  effects  are  the  result  of  the  subsequent  reactions  in 
which  these  ions  are  involved. 

Induced  mutations  are  in  general  similar  to  spontaneous  mutations, 
though  chromosome  breakage  is  relatively  more  frequent  among  the  induced 
mutations.  The  vast  majority  of  induced  mutations  are  recessive  and  deleterious 
under  existing  conditions.  There  is  no  threshold  dose  of  radiation  below  which 
no  mutations  are  induced.  Any  increase  in  ionizing  radiation  above  the  back- 
ground can  therefore  be  expected  to  cause  a  corresponding  increase  in  the  num- 
ber of  mutations.  The  mutation  rate  has  been  shown  to  be  directly  proportional 
to  the  dosage  of  radiation.  A  doubling  of  the  dosage  will  result  in  a  doubling  in 
the  number  of  induced  mutations.  For  chromosomal  rearrangements  such  as  in- 
versions or  reciprocal  translocations,  however,  the  number  of  rearrangements 


10- 


Fig.  34-2.  Relation  between  x-ray  dosage  and  the  frequency  of  one-hit  and 
two-hit  chromatid  rearrangements.  One-hit  rearrangements  increase  in  direct 
proportion  to  the  dosage  but  two-hit  rearrangements  tend  to  increase  as  the 
square  of  the  dosage  at  relatively  high  intensities.    (With  permission  of  Sax.) 


RADIATION,   GENETICS,   AND   MAN  •  355 

increases  more  nearly  as  the  square  of  the  dosage.  This  difference  is  attributed 
to  the  fact  that  two  independent  breaks  are  required  for  the  rearrangements, 
whereas  mutations  are  single-hit  events.  See  Fig.  34-2. 

Although  an  intensity  effect  has  recently  been  reported  in  mice,  the 
number  of  gene  mutations  induced  is  usually  independent  of  the  intensity 
with  which  the  radiation  is  delivered.  One  hundred  roentgens  in  5  minutes 
causes  the  same  number  of  point  mutations  as  lOOr  in  5  months  or  5  years.  A 
roentgen  of  x-  or  gamma  radiation  is  the  amount  that  will,  when  applied  to  air 
at  standard  conditions  (0°  C,  760  mm  mercury),  produce  2.1  X  109  ion  pairs 
per  cubic  centimeter  (1  electrostatic  unit  of  charge).  In  water  or  tissue,  the 
number  of  ion  pairs  produced  by  If  is  estimated  to  be  about  800  times  greater. 
Much  easier  to  remember  is  the  fact  that  It  causes  approximately  2  ionizations 
per  cubic  micron  of  tissue.  The  effect  of  the  radiation,  then,  is  cumulative,  for  a 
mutation,  once  it  occurs,  does  not  heal,  but  is  self-duplicating  and  persists  until 
it  causes  a  genetic  death. 


Somatic  Effects  of  Radiation 

A  severe  exposure  to  radiation  may  be  lethal.  The  acute  lethal  dose  for 
50  percent  of  the  exposed  individuals  (the  so-called  L.D.  50)  has  been  estimated 
in  man  to  be  in  the  range  of  from  400  to  600r.  Lesser  doses  produce  a  variety  of 
somatic  effects,  and  the  parts  of  the  body  where  cell  division  is  rapidly  occurring 
appear  to  be  particularly  sensitive.  Early  symptoms,  for  example,  among  the  sur- 
vivors of  the  explosions  at  Hiroshima  and  Nagasaki  were  disturbances  of  the 
gastrointestinal  tract  and  in  the  blood-forming  tissues.  Temporary  or  in  some 
instances  permanent  sterility  may  be  induced.  Later  effects  of  acute  exposure  or 
of  low-level  chronic  exposure  include  skin  cancer  and  leukemia,  which  may  not 
develop  until  long  after  the  exposure.  Finally,  in  addition  to  these  rather  specific 
ailments,  there  are  nonspecific  effects  such  as  a  lower  immunity  to  disease, 
damage  to  the  connective  tissue,  and  signs  of  premature  aging.  In  mice,  the 
most  sensitive  index  of  somatic  damage  is  the  shortening  of  the  life  span. 

All  of  the  biological  effects  of  radiation  mentioned  thus  far  have  been 
observed  in  man  except  for  one,  the  induction  of  gene  mutations.  Skin  cancer, 
leukemia,  etc.,  are  known  consequences  of  exposure  to  radiation  that  have  been 
made  more  or  less  familiar  by  newspaper  reports.  Shortening  of  the  life  span  is 
indicated  by  data  on  radiologists  (Table  34-1).  Even  the  breakage  of  human 
chromosomes  has  been  demonstrated  in  human  cells  in  tissue  culture  by  Bender. 
In  view  of  these  effects  in  man,  there  is  no  reason  to  suppose  that  he  has  some 
sort  of  mysterious  immunity  to  the  mutagenic  effects  of  radiation.  Why,  then, 
has  it  not  yet  been  demonstrated  ? 


356  •  EVOLUTION   AND   MAN 


TABLE    34-1 
Effects  of  Radiation  on  Life  Span 


Group  Average  age  at  death 

U.S.  population  over  25  65.6 

Physicians  (not  exposed)  65-7 

Physicians  (some  exposure — urologists,  dermatologists,  etc.)  63.3 

Radiologists  60.5 

*  National  Academy  of  Sciences,  1956.  "The  biological  effects  of  atomic  radiation,"  Summary 
reports.  Washington,  D.  C. 


Radiation  Effects  in  Man 

The  largest  study  of  the  genetic  effects  of  radiation  was  made  under 
the  auspices  of  the  United  States  Atomic  Energy  Commission  on  the  children  of 
the  survivors  of  the  explosions  at  Hiroshima  and  Nagasaki.  Among  these  chil- 
dren, as  compared  to  the  controls,  there  were  no  statistically  significant  increases 
in  the  number  of  stillbirths  or  abnormalities  but  a  possible  slight  effect  on  the 
sex  ratio  was  reported.  The  Genetics  Conference  that  set  up  the  project  expected 
these  results  from  the  outset,  but  the  opportunity  for  such  a  study  was  unique 
and  it  seemed  wise  to  seize  it. 

Let  us  consider  the  reasons  for  the  lack  of  significant  differences  be- 
tween exposed  and  control  populations.  The  unexposed  controls  showed  more 
than  1  percent  visible,  though  slight,  malformations  at  birth,  a  part  of  the  4  to 
5  percent  defective  mentioned  previously.  Furthermore,  less  than  1  in  100  in- 
duced mutants  are  dominant  and  will  be  immediately  expressed  in  the  next 
generation.  Therefore,  it  has  been  estimated  that  out  of  1000  children  whose 
parents  both  received  lOOr  (that  is,  the  most  heavily  irradiated  survivors),  it 
can  be  expected  that  30  percent  will  carry  a  newly  induced  mutation.  However, 
only  about  1  percent  of  these  mutations  will  be  dominant  and  expressed  in  the 
children.  Simple  arithmetic  (1000  X  0.3  X  0.01  =  3)  shows  that  only  3  among 
the  1000  childen  can  be  expected  to  be  malformed  at  birth  due  to  the  irradiation. 
Since  1  percent  or  10  in  1000  can  be  expected  to  be  malformed  at  birth  due  to 
causes  other  than  the  radiation,  a  statistical  comparison  is  required  between  10  in 
1000  and  13  in  1000.  Obviously,  such  a  small  difference  will  be  subject  to  ran- 
dom fluctuations  unless  very  large  numbers  are  available  for  study.  Special 
genetic  techniques  that  are,  fortunately,  not  available  to  the  human  geneticist 
would  be  needed  to  reveal  the  much  more  numerous  induced  recessives.  How- 
ever, lack  of  the  techniques  is  no  reason  to  suppose  that  mutations  have  not  oc- 
curred and  been  added  to  the  existing  load  of  mutations.  Other  studies  of  the 
children  of  radiologists  and  of  children  whose  parents  have  received  therapeutic 
pelvic  irradiation  of  lOOOr  or  more  (skin  dose)  have  indicated  a  genetic  effect. 


RADIATION,    GENETICS,    AND    MAN  •   357 

These  data  were  assembled  by  questionnaire  and  are  possibly  subject  to  bias  since 
the  returns  were  not  complete.  One  conclusion  that  can  be  drawn,  perhaps,  from 
the  genetic  studies  is  that  man  cannot  be  much  more  susceptible  to  radiation  than 
are  mice. 

A  useful  way  to  look  at  the  problem  is  in  terms  of  the  doubling  dose, 
that  amount  of  radiation  which  will  induce  as  many  mutations  as  now  occur 
spontaneously.  The  doubling  dose  was  independently  estimated  by  two  groups 
in  the  United  States  and  Great  Britain  with  surprisingly  good  agreement  as  50r 
and  30  to  80r.  In  other  words,  if  the  population  of  the  United  States  were  sub- 
jected to  an  additional  50r  per  generation,  the  number  of  children  born  with 
genetic  defects  would  gradually  rise  from  2  percent  to  4  percent  as  the  new 
equilibrium  is  reached.  Taking  all  factors  into  account,  the  National  Academy  of 
Sciences  has  recommended  that  the  total  accumulated  dose  of  ionizing  radiation 
from  humanly  controllable  sources  to  the  reproductive  cells  from  conception  to 
age  30  should  not  be  more  than  lOr.  This  recommended  dose  is  by  no  means 
harmless  but  is  considered  reasonable.  However,  for  100  million  children  an 
increase  of  lOr  is  estimated  to  give  rise  to  50,000  new  inherited  defects  in  the 
first  generation  and  ultimately  at  the  new  equilibrium  to  500,000  per  generation. 
Clearly,  any  increase  at  all  must  be  regarded  as  harmful.  Recent  estimates  for  the 
average  exposure  to  radiation  of  the  gonads  of  the  population  of  the  United 
States  are  as  follows: 

Source  of  radiation 

background  3.1r  per  30  years 

medical  uses  of  radiation  4.6r  per  30  years 

fallout  from  atomic  explosions  O.lr  per  30  years 

There  apparently  is  a  threshold  for  most  somatic  effects  of  radiation, 
for  with  two  possible  exceptions,  doses  several  times  as  large  as  the  recom- 
mended lOr  limit  are  necessary  to  cause  detectable  somatic  damage.  One  possible 
exception  is  the  shortening  of  the  life  span.  Even  though  doses  of  up  to  lOOr 
spread  over  a  period  of  years  have  not  been  shown  to  shorten  human  life,  it  is 
still  possible  that  there  is  no  threshold.  If,  for  example,  large  numbers  of  people 
exposed  to  a  gradually  accumulated  dose  had  their  life  expectancy  lowered  very 
slightly,  the  individual  effect  might  seem  trivial,  but  the  total  effect  would  be 
very  great. 

The  other  possible  exception  is  the  effect  of  strontium-90.  This  radio- 
active element,  rather  similar  chemically  to  calcium,  tends  to  accumulate  in  bone. 
The  major  hazard  from  Sr90  is  the  internal  radiation  of  the  red  bone  marrow, 
which  may  lead  to  the  development  of  leukemia.  The  maximum  permissible  con- 
centration (MPC)  of  Sr90  in  man  has  been  set  at  1  microcurie  per  1000  grams 
of  calcium.  (A  microcurie  produces  an  amount  of  radiation  equivalent  to  that 
emanating  from  a  millionth  of  a  gram  of  radium.  The  body  of  the  average 


-c 


358  •  EVOLUTION   AND    MAN 

human  adult  contains  about  1000  grams  of  calcium.)  Just  0.1  of  the  MPC  would 
give  a  dose  rate  of  0.1  to  0.2r  per  year  to  the  red  bone  marrow.  For  the  present 
population  of  the  United  States,  the  expected  number  of  additional  cases  of 
leukemia  at  this  dosage  level  would  be  500  to  1000  per  year.  Since  there  are 
currently  about  10,500  deaths  from  leukemia  in  the  United  States  each  year,  one- 
tenth  the  MPC  of  Sr90  would  be  expected  to  increase  the  present  incidence  of 
leukemia  about  5  or  10  percent.  However,  the  present  levels  of  Sr90  in  bone  are 
about  1/1000  rather  than  1/10  of  the  MPC,  and  therefore  Sr90  cannot  now  be 
regarded  as  a  major  hazard  to  the  human  population;  the  level  of  Sr90  in  bone 
must  be  watched,  however,  for  if  it  rises,  the  hazard  will  increase.  Furthermore, 
it  seems  unlikely  that  the  existing  levels  of  exposure  are  causing  any  major 
shortening  in  the  human  life  span.  However,  there  is  no  question  that  much 
additional  research  is  needed  to  back  up  the  available  estimates  and  to  clarify 
still  further  the  somatic  effects  of  radiation. 

The  major  hazard  at  the  present  time  is  the  genetic  effect  of  radiation, 
and  the  major  source  of  man-made  radiation  for  the  population  of  the  United 
States  is  the  medical  use  of  radiation.  The  amount  received  currently  from  fall- 
out is  only  1  or  2  percent  as  great  as  the  amount  received  in  the  course  of  the 
various  medical  uses  of  ionizing  radiation.  While  some  scientists  have  greatly 
emphasized  the  dangers  inherent  in  nuclear  weapons  testing,  others  equally 
reputable  have  suggested  that  the  dangers  are  trivial  or  nonexistent  or  may  even 
be  beneficial.  Under  these  circumstances  the  public  cannot  be  blamed  for  being 
somewhat  confused  about  the  hazards  involved.  A  true  concern  for  human  wel- 
fare would  seem  to  dictate  that  the  problem  of  radiation  hazard  must  be  faced 
as  a  whole,  and  that  the  solution  must  encompass  not  only  nuclear  tests  but  the 
medical  and  industrial  uses  of  radiation  as  well.  The  evidence  now  available 
indicates  quite  clearly  that  the  net  effect  of  any  increase  in  the  exposure  of  the 
human  population  to  radiation  will  be  harmful.  However,  it  is  also  clear  that 
more  research  is  desirable  and  necessary  to  delineate  more  specifically  just  how 
great  are  the  hazards  to  man. 

The  varied  uses  of  radiation  raise  questions  to  which  there  are  no  simple 
answers.  For  the  physician,  each  use  of  radiation  requires  that  he  weigh  the  im- 
mediate benefits  to  his  patient  against  the  possible  genetic  damage  to  future  gen- 
erations. And  this,  of  course,  raises  the  question  of  just  what  are  our  obligations 
to  future  generations.  Is  it  possible  that  the  doctors  of  another  day  will  be  able 
to  mend  damaged  genes  as  they  now  mend  broken  legs?  If  it  is  possible,  how 
much  radiation  can  the  human  species  safely  absorb  until  that  day  comes?  The 
weapons  tests  similarly  require  an  evaluation  of  the  benefits  and  hazards  of  test- 
ing versus  not  testing.  Unfortunately,  the  decisions  on  testing  are  based  in  the 
final  analysis  on  political  rather  than  on  scientific  or  humanitarian  considerations. 


RADIATION,    GENETICS,    AND    MAN  •  359 

SUGGESTED  READING 

Effect  of  radiation  on  human  heredity,  1957.  Geneva:  World  Health  Organization. 

"Ionizing  radiation,"  Scientific  American,  201(5)  Sept.  1959. 

Medical  Research  Council,  1956.  "The  hazards  to  man  of  nuclear  and  allied  radia- 
tions," Cmd.  9780.  London:  H.  M.  Stationery  Office.  2d  Report,  I960. 
Cmnd.  1225. 

Muller,  H.  J.,  1950.  "Radiation  damage  to  the  genetic  material,"  Amer.  Scientist, 
38:33-59;  399-425. 

,  1950.  "Our  load  of  mutations,"  Amer.  Jour.  Human  Genetics,  2:111-176. 

National  Academy  of  Sciences,  1956.  "The  biological  effects  of  atomic  radiation," 
summary  reports.  Washington,  D.C.  2d  Report,  I960. 

Wallace,  B.  and  Th.  Dobzhansky.  1959.  Radiation,  genes,  and  man.  New  York: 
Holt,  Rinehart  and  Winston. 


CHAPTER 


35 


Man  as  a  Dominant  Species 


The  human  population  is  subject  to  the  effects  of  natural 
selection,  mutation,  gene  flow,  and  random  genetic  drift  just  as 
are  the  populations  of  other  species.  In  the  future  as  in  the  past, 
the  qualitative  characteristics  of  the  human  population  during  the 
course  of  its  evolution  will  be  determined  by  the  net  effect  of  the 
action  of  these  factors.  However,  in  addition  to  changing  qualita- 
tively, the  human  population  may  also  change  quantitatively.  The 
most  noteworthy  aspect  of  human  biology  in  the  last  few  centuries 
has  been  the  tremendous  increase  in  the  size  of  the  human  popu- 
lation, an  increase  of  such  overriding  significance  that  any  con- 
sideration of  human  affairs  that  fails  to  include  it  is  seriously 
deficient. 

The  population  problem  is  an  involved,  controversial, 
and  paradoxical  subject,  so  beset  by  emotion  and  prejudice  that 
discussing  it  objectively  is  far  more  difficult  than  discussing  fac- 
tors that  regulate  the  numbers  of  grasshoppers  or  deer  or  field 
mice.  There  are  two  schools  of  thought  about  the  hazards  of 
man's  increasing  numbers.  One  group  will  state  flatly  that  Malthus 
has  long  since  been  proven  wrong,  that  man  can  produce  all  the 
food  and  goods  necessary  for  any  possible  increase  in  his  numbers, 
and  that  his  ingenuity  and  resourcefulness  (or  science  and  tech- 
nology) will  insure  that  production  will  more  than  keep  pace 
with  population  growth.  Any  present  difficulties  in  getting  suffi- 
cient food  and  other  necessities  are  attributed  to  a  failure  in  the 
system  of  distribution  rather  than  to  overpopulation.  One  cannot 
help  but  wonder  at  times  whether  these  people  have  ever  read  the 

360 


MAN   AS   A   DOMINANT   SPECIES  •  361 

words  of  Malthus  whom  they  so  readily  dismiss.  Opposed  to  this  group 
is  another  group,  who  will  point  out  that  right  now  three-fifths  of  the 
world's  people  are  living  at  a  bare  subsistence  level,  and  that  since  we 
can  not  even  take  care  of  our  present  population  in  a  satisfactory  way,  there 
is  no  reason  to  suppose  that  we  can  do  so  in  the  future  if  the  present  rate  of 
increase  continues.  The  question  is  whether  the  earth's  resources  are  sufficient  to 
support  the  present  population  and  the  potential  future  population  at  a  standard 
of  living  above  the  bare  subsistence  level.  The  future  of  mankind  may  well  hinge 
more  on  the  answer  to  this  question  than  on  any  other  single  factor. 

In  order  to  make  an  objective  appraisal  of  the  pros  and  cons  of  this 
question,  certain  relevant  facts  must  be  reviewed.  All  living  organisms,  including 
man,  are  ultimately  dependent  for  their  very  existence  on  the  photosynthetic 
processes  of  green  plants  by  which  the  sun's  energy  is  utilized  to  form  organic 
materials  (that  is,  food)  from  simple  inorganic  compounds.  This  fact  is  inescap- 
able at  the  present  time,  and  it  appears  unlikely  that  other  means  of  synthesizing 
food  in  significant  quantities  will  be  devised  in  the  near  future.  The  maximum 
size  of  the  human  population,  then,  ultimately  depends  on  the  amount  of  food 
that  can  be  grown  to  support  it.  The  areas  available  on  the  earth  in  which  food 
might  be  grown  consist  of  the  following: 

A.  Land  1.  Fertile  regions  33,000,000  square  miles 

2.  Steppes  19,000,000  square  miles 

3.  Deserts  5,000,000  square  miles 

B.  Water  140,000,000  square  miles 

This  is  all  there  is;  there  isn't  any  more.  (The  implications  of  the  space  age  can 
safely  be  ignored  in  the  present  discussion,  for  the  problems  of  transportation 
and  distribution  have  not  yet  been  successfully  solved  here  on  earth  and  will  be 
infinitely  greater  in  any  interplanetary  situation.)  Crops  can  only  be  raised  in  the 
fertile  regions.  The  vegetation  of  the  steppes  is  made  available  to  man  through 
its  use  as  pasture;  the  vegetation  of  the  seas  is  the  pasture,  in  a  sense,  of  the 
fishes.  The  amount  of  fertile  land  can  be  increased  through  irrigation.  The  yield 
can  be  improved  through  improved  agricultural  methods  and  the  use  of  im- 
proved varieties  of  plants  and  animals.  These  changes  have  been  and  are  con- 
tinuing to  be  made  in  many  parts  of  the  earth  with  spectacular  success  in  many 
instances  in  increasing  the  productivity  of  the  land. 

Man  has  existed  for  at  least  several  hundred  thousand  years.  Although 
exact  figures  are  not  available,  the  best  estimates  indicate  that  until  1650  human 
population  growth  was  relatively  slow  and  erratic.  By  that  time  the  human  popu- 
lation was  estimated  to  be  about  500  million.  In  less  than  200  years,  by  1825, 
world  population  had  doubled,  and  for  the  first  time  more  than  a  billion  people 


362   •  EVOLUTION    AND    MAN 

inhabited  the  earth.  In  another  100  years  the  population  had  again  doubled  to 
2  billion.  In  the  few  decades  since  1925  this  growth  has  continued,  until  the 
present  world 'population  is  estimated  to  be  over  2.8  billion  people.  Thus  from  a 
species  of  limited  range  and  numbers,  man  has  seemed  almost  literally  to  explode 
over  the  face  of  the  earth.  He  is  now  a  cosmopolitan  species,  yet  it  seems  likely 
that  50,000  years  ago  North  and  South  America  were  completely  uninhabited  by 
man,  and  that  in  the  inhabited  areas  the  population  density  was  low,  typical  of  a 
hunting  or  nomad  population.  See  Fig.  35-1. 


Millions 


lining 


World  population  growth 
5000  BC-1950  AD 


I  l  I  I  M  I  I  I 


3500 

--3000 
-- 2500 

--2000 

1500 

1000 

--500 
■•   100 


Year 


4000 


3000 


2000 


1000  BC 


1  AD 


1000 


2000 


Fig.  35-1.     World  population  growth,  5000  B.C.  to  1950  a.d.    Not  only  the  size 

of  the  human  population  but  the  annual  rate  of  growth  has  increased  markedly 

since   1800.   (With  permission  of  Sax.) 


Not  only  has  the  human  population  increased,  but  it  has  increased  at  an 
accelerating  rate.  The  annual  rate  of  increase  has  grown  from  an  estimated 
0.4  percent  between  1650  and  1850  to  0.8  percent  between  1850  and  1950,  and 
is  currently  estimated  to  be  about  1.7  percent  per  year.  The  numerical  increase  is 
thought  to  be  nearly  45  million  a  year  or  about  123,000  per  day.  Projection  of 
these  figures  into  the  future  has  led  to  estimates  of  6  billion  people  by  the  year 
2000  and  nearly  13  billion  by  2050.  The  facts  are,  then,  that  we  have  on  the 
earth  a  limited  amount  of  space  and  fertile  land  on  which  to  support  a  human 
population  rapidly  growing  at  an  accelerating  pace.  Obviously  this  growth  cannot 
and  will  not  continue  indefinitely  and  these  figures  may  never  be  reached.  How- 


MAN   AS   A   DOMINANT   SPECIES  •  363 

ever,  the  way  in  which  this  trend  is  slowed  or  reversed  will  have  a  tremendous 
impact  on  the  future  welfare  and  happiness  of  mankind. 

Elementary  Demography 

Under  favorable  conditions,  the  human  population  could  easily  double 
every  25  years.  The  fact  that  it  has  not  done  so  is  an  indication  that  man's  exist- 
ence has  been  rather  precarious,  with  disease,  pestilence,  famine,  natural  catastro- 
phes, and  war  all  having  exacted  a  heavy  toll  in  the  past.  The  size  of  any  popu- 
lation is  determined  by  the  relationship  between  the  death  rate  and  the  birth 
rate,  and  even  though  birth  rates  were  high  in  the  past,  death  rates  were  also 
high  so  that  growth  of  the  human  population  was  slow  and  irregular.  The  most 
common  way  to  express  birth  rates  or  death  rates  is  in  terms  of  the  number  of 
births  or  deaths  per  1000  population,  the  so-called  crude  birth  and  death  rates. 
Since  both  birth  rates  and  death  rates  vary  with  age,  the  crude  rates  will  also 
depend  on  the  age  structure  of  the  population  and  may  not  be  directly  com- 
parable in  two  populations  having  different  age  distributions. 

The  rapid  population  growth  in  the  Western  world  during  the  last  few 
centuries  has  been  due  to  the  scientific  revolutions  in  the  fields  of  public  health, 
agriculture,  and  industry.  The  initial  effect  of  these  revolutions  was  a  reduction 
in  the  death  rate,  and  this  can  be  attributed  primarily  to  the  revolution  in  medi- 
cine and  public  health.  Many  diseases  have  been  eliminated  or  brought  under 
control  so  that  infant  mortality  has  been  reduced  from  about  200  to  about  30  per 
thousand  infants  and  the  average  crude  death  rate  has  fallen  from  about  40  to 
about  12  or  less  per  thousand.  As  a  consequence,  life  expectancy  at  birth  has 
risen  from  between  25  and  30  to  between  60  and  70  years. 

The  revolution  in  agriculture  has  resulted  from  mechanization  and  from 
scientific  advances  in  plant  and  animal  breeding  as  well  as  in  the  methods 
of  cultivation  and  fertilization;  yield  per  acre  and  also  yield  per  agricultural 
worker  have  risen  dramatically  among  the  Western  nations.  In  the  United  States 
in  1700,  for  example,  it  took  4  farm  families  to  produce  enough  food  for  5  fam- 
ilies. Today  one  farm  family  produces  enough  food  for  6  families,  or  for  10 
families  living  at  the  standards  of  1700.  Therefore,  since  the  efforts  of  5  out  of 
6  families  can  now  be  diverted  from  the  production  of  food  into  the  production 
of  other  goods  and  services,  the  standard  of  living  has  risen  rapidly. 

The  industrial  revolution,  which  went  more  or  less  hand  in  hand  with 
the  agricultural  revolution,  increased  the  food  supply  through  the  mechanization 
of  farming  and  through  the  improved  transportation  system,  by  which  food 
could  be  shipped  from  areas  of  high  production  to  areas  of  consumption  where 
it  was  exchanged  for  manufactured  products.  Emigration  from  crowded  regions 
in  Europe  to  empty  lands  in  America  and  elsewhere  overseas  became  possible, 
and  helped  to  relieve  the  pressure  of  an  expanding  population. 


364  •  EVOLUTION    AND    MAN 


Still  another  transition  has  taken  place  in  most  of  the  Western  nations, 
perhaps  as  revolutionary  as  any  thus  far  mentioned.  This  revolution,  more  recent 
in  onset  than  the  others,  has  resulted  in  declining  birth  rates.  As  a  result  of  the 
time  lag  between  the  fall  in  the  death  rates  and  the  fall  in  the  birth  rates,  the 
so-called  demographic  transition  from  a  high  birth  rate-high  death  rate  agri- 
cultural society  to  a  low  birth  rate-low  death  rate  industrial  society  has  always 
historically  been  accompanied  by  a  rapid  increase  in  population  size  (see  Fig. 
35-2).  When  the  death  rate  is  lower  than  the  birth  rate,  the  difference  between 
the  two  can  be  regarded  as  a  measure  of  the  net  increase;  when  the  birth  rate 
falls  below  the  death  rate,  the  population  will,  if  this  relation  persists,  decline  in 
numbers. 


STAGE  I 

High  Fluctuating 

(HIGH  BIRTH  AND     ', 
DEATH  RATES)  ^ 


STAGE  II 

Early  Expanding 

(HIGH  BIRTH  RATE5,DECLINING  DEATH  RATES) 


STAGE  III    !     STAGE IV 

Late  Expanding  !  Low  Fluctuating 

(DECLINING  BIRTH  !(LOW  BIRTH  AND 
AND  DEATH  RATES);    DEATH  RATES) 


Rate  per 
thousand 


Dates 


Population       6*5 

(miuions) 


1800 
9 


1850 

1880 

1900 

1930 

1950 

18 

26 

32-5 

40 

43-5 

Fig.  35-2.     The  demographic  transition  in  England  and  Wales  from  a  society 

with  high  birth  and  death  rates  to  one  with  low  rates  of  births  and  deaths. 

(With  permission  of  P.E.P.  Report.   World  Population  and  Resources.) 

During  the  demographic  transition,  various  stages  can  be  recognized. 
Initially  there  is  an  agricultural  society,  with  high  birth  rates,  high  death  rates,  a 
slow  and  irregular  increase  in  numbers,  and  a  relatively  low  standard  of  living. 
In  the  next  stage  the  death  rate  starts  to  fall  quite  rapidly  while  the  birth  rate 
continues  high.  The  decline  in  the  death  rate  comes  first  because  the  measures 
needed  to  control  the  death  rate  are  relatively  simple  and  easy  to  put  into  effect. 
Thus  the  initial  impact  of  modern  scientific  knowledge  on  a  backward  society 
has  been  on  the  death  rate.  The  sensitivity  of  the  death  rate  to  changed  condi- 


MAN    AS   A    DOMINANT   SPECIES  •   365 

tions  can  be  illustrated  by  the  spectacular  drop  in  Japan  from  a  death  rate  of 
32  per  1000  in  1945  to  12  per  1000  in  1948.  In  Ceylon  an  antimalarial  cam- 
paign using  DDT  brought  the  death  rate  from  20.2  in  1946  down  to  14.2  in 
1947  and  to  9-8  by  1956.  In  1946  there  were  12,578  deaths  from  malaria;  in 
1947,  4557;  and  in  1956,  144.  Life  expectancy  at  birth  rose  from  45.8  years  to 
over  60  years.  Obviously  when  death  rates  decline  in  this  fashion  and  a  decline 
in  birth  rates  does  not  immediately  follow,  the  population  increase  is  rapid. 

During  the  next  phase,  the  birth  rate  also  starts  to  decline  rather  rapidly 
while  the  death  rate  continues  to  fall.  The  causes  of  declining  birth  rates  have 
never  been  clearly  defined,  but,  and  here  is  the  paradox,  birth  rates  have  started 
to  fall  in  the  past  only  after  standards  of  living  have  improved.  Thus,  birth  rates 
are  highest  in  just  those  areas  where  people  are  least  able  to  support  large  fam- 
ilies. During  this  period  when  both  birth  and  death  rates  are  declining,  popula- 
tion continues  to  increase  but  at  a  decelerating  pace. 

The  final  stage,  reached  when  the  demographic  transition  is  completed, 
is  marked  by  low  birth  and  death  rates  and  near  equilibrium  conditions.  Usually 
the  birth  rate  remains  somewhat  higher  than  the  death  rate  so  that  the  population 
continues  to  grow  at  a  slow  rate.  In  countries  that  have  made  the  transition, 
standards  of  living  are  high,  life  expectancy  is  long,  and  birth  rates  are  very 
sensitive  to  economic  forces. 

Burma  may  be  cited  as  an  example  of  an  underdeveloped  nation  with 
high  birth  rates  (47.5,  1951-53),  high  death  rates  (35.7,  1951-53),  and  a  rather 
slow  rate  of  growth  (though  declining  death  rates  may  lead  to  more  rapid 
growth).  The  island  of  Mauritius  appears  to  have  reached  the  second  stage,  since 
the  birth  rate  (1949-53)  was  46.5  while  the  death  rate  was  only  14.9,  the  result 
being  a  sizable  natural  increase.  Puerto  Rico  has  recently  reached  the  third  phase 
of  the  demographic  transition,  for  her  birth  rate  had  fallen  (1953)  from  be- 
tween 40  and  50  to  34.8  and  the  death  rate  to  8.1.  The  United  Kingdom  has 
essentially  completed  the  change,  for  in  1953  the  birth  rate  was  15.9  and  the 
death  rate  was  11.4,  and  the  rate  of  natural  increase  was  quite  low. 

In  Western  Europe  the  change  from  a  high  birth  rate-high  death  rate 
society  to  a  low  birth  rate-low  death  rate  society  brought  about  a  sixfold  increase 
in  population.  North  America  had  a  sixfold  increase  in  just  a  single  century, 
between  1850  and  1950.  Japan  has  made  the  transition  in  less  than  a  century, 
more  rapidly  than  any  other  nation,  and  yet,  despite  the  speed  of  the  change, 
has  almost  tripled  from  about  35  million  in  1868  to  nearly  100  million  today. 

In  spite  of  their  increases  in  population  size,  the  nations  of  the  Western 
world  have  had  a  notable  rise  in  their  standards  of  living.  The  economic  well- 
being  of  the  people  of  these  nations  is  higher  than  it  has  ever  been  anywhere. 
This  fact,  that  standards  of  living  have  increased  while  populations  were  grow- 
ing rapidly,  has  led,  it  seems  clear,  to  the  optimistic  view  that  Malthus  was 
wrong.  However,  his  basic  statements  were: 


f 


366  •  EVOLUTION   AND   MAN 

1.  Population  is  necessarily  limited  by  the  means  of  subsistence.  2.  Popula- 
tion invariably  increases,  where  the  means  of  subsistence  increase,  unless  prevented 
by  some  very  powerful  and  obvious  checks.  3.  These  checks,  and  the  checks  which 
repress  the  superior  power  of  population,  and  keep  its  effects  on  a  level  with  the 
means  of  subsistence  are  all  resolvable  into  moral  restraint,  vice  and  misery. 

He  distinguished  between  preventive  checks,  which  tended  to  reduce  the 
birth  rate,  and  positive  checks,  which  raised  the  death  rate.  The  essential  sound- 
ness of  his  position  seems  clear.  What  he  did  not  foresee  was  the  possibility  that 
preventive  checks  could  come  to  be  as  significant  as  they  are  in  some  nations 
today. 


The  Causes  of  Overpopulation 

Because  of  the  revolutions  in  agriculture  and  industry,  the  means  of  sub- 
sistence in  the  Western  world  have  increased  even  more  rapidly  than  has  the 
population,  and  the  West  has  managed  thus  far  to  escape  the  Malthusian  devil 
of  overpopulation.  The  meaning  intended  here  for  the  term  "overpopulation" 
is  that  there  are  more  people  than  can  be  supported  at  a  reasonable  standard  of 
living  on  the  available  resources  (of  all  kinds)  in  the  area.  The  implication  is 
that  if  the  population  had  not  grown  so  large,  the  people  individually  would  be 
better  off,  and  if  it  continues  to  grow,  living  standards  will  fall  still  further.  It 
will  be  worthwhile  to  examine  the  routes  by  which  the  West  has  escaped  this 
situation  and  to  evaluate  their  applicability  to  those  areas  of  the  world  that  have 
yet  to  make  the  transition. 

When  such  an  analysis  is  made,  it  becomes  obvious  that  the  three-fifths 
of  the  world's  people  who  have  a  low  living  standard  (per  capita  income  usually 
less  than  $100  per  year),  an  average  length  of  life  in  the  30's,  a  high  birth  rate, 
and  a  low  literacy  rate  cannot  hope  to  escape  from  overpopulation  by  following 
the  same  sequence  of  events  as  the  Western  world.  This  statement  may  seem 
rather  dogmatic  and  therefore  warrants  further  more  detailed  consideration  and 
discussion. 

The  first  impact  of  modern  scientific  knowledge  on  a  backward  agricul- 
tural society  has  always  been  on  the  death  rate,  because  public  health  measures 
such  as  sewage  disposal,  water  purification,  mosquito  control,  vaccination,  etc., 
are  relatively  inexpensive  and  easy  to  institute.  However,  the  longer  life  will  not 
necessarily  be  a  happier  one,  for  countries  such  as  India,  China,  and  Egypt  are 
already  densely  populated  and  cannot  hope  to  support  even  a  twofold  increase  in 
population  at  a  higher  standard  of  living,  let  alone  a  threefold,  or  sixfold,  or 
tenfold  increase.  Efforts  toward  industrialization  are  beset  by  the  fact  that  there 
are  few  areas  (the  United  States  is  one)  with  excess  food  to  exchange  for  manu- 
factured products,  and  these  areas  may  not  need  or  want  the  manufactured  goods. 
Furthermore,  markets  are  not  as  readily  available  as  they  were  150  years  ago.  If 


MAN   AS  A   DOMINANT   SPECIES  •  367 

it  is  argued  that  the  primary  need  is  to  increase  agricultural  production  rather 
than  industrial  expansion,  another  dilemma  presents  itself.  This  situation  can 
best  be  clarified  by  an  actual  example.  It  might  be  expected  that  a  marked  rapid 
increase  in  the  food  supply  would  give  the  farmers  a  surplus  that  could  then  be 
exchanged  for  manufactured  goods  and  for  services  so  that  their  living  standards 
would  rise.  This  argument,  in  one  form  or  another,  seems  to  be  the  one  that  has 
led  some  individuals  to  view  the  population  problem  with  equanimity.  However, 
it  ignores  the  demographic  effects  of  an  increased  food  supply  in  an  under- 
developed country,  and  therein  lies  its  fallacy.  On  the  Malabar  coast  of  India, 
rice  had  been  the  staple  food  crop  for  centuries  until  research  showed  that  tapioca 
(cassava)  was  a  more  profitable  crop  for  this  area.  The  change  to  tapioca  was 
put  into  effect  rapidly,  and  food  production  was  approximately  doubled  in  a  few 
years.  In  just  12  years,  however,  the  population  in  this  area  had  also  doubled,  so 
that  twice  as  many  people  now  lived  on  twice  as  much  food  at  the  same  bare 
subsistence  level.  Therefore,  even  though  various  governments  have  set  up  five- 
year  plans  or  other  programs  designed  to  increase  agricultural  production  or  to 
encourage  the  development  of  industry,  such  programs  may  not  resolve  the  prob- 
lems, even  when  their  goals  are  achieved,  if  the  demographic  factors  are  not 
favorable  or  are  ignored  in  the  planning.  In  fact,  the  situation  may  actually  be- 
come worse  than  before. 

The  pressure  of  the  growing  population  in  Europe  during  the  demo- 
graphic transition  was  relieved  in  part  by  the  emigration  of  large  numbers  of 
Europeans  to  America  and  to  other  parts  of  the  world  that  were  then  sparsely 
populated.  The  safety  valve  provided  by  emigration  is  no  longer  available,  for 
there  are  no  more  large  unoccupied  habitable  areas  in  the  world.  Furthermore, 
the  very  magnitude  of  the  logistic  problems  involved  makes  it  clear  that  the 
solution  for  overpopulated  areas  is  not  to  export  their  surplus  population  (even 
if  they  were  able  to  decide  who  was  surplus  and  who  was  not).  In  India,  for 
example,  a  series  of  favorable  crop  years  between  1931  and  1941  led  to  an 
increase  of  50  million  in  her  population,  an  average  of  5  million  per  year. 
Imagine,  if  you  will,  the  problems  involved  merely  in  transporting  5  million 
people  per  year  from  India  to  some  other  part  of  the  world,  not  to  mention  the 
problems  of  finding  housing  and  jobs  for  them  in  their  new  environment.  The 
United  States,  at  the  peak  of  its  all-out  effort  in  World  War  II,  transported  and 
supported  overseas  only  about  8  million  men.  Clearly,  the  relocation  of  millions 
of  people,  the  numbers  about  which  we  must  think,  would  be  impossible,  espe- 
cially for  those  nations  whose  resources  are  already  strained  by  overpopulation. 
Hence,  this  solution  holds  little  promise  for  the  present  problems  even  if  un- 
developed lands  were  available.  A  further  complication  should  also  be  pointed 
out  to  indicate  another  aspect  of  the  problems  created  by  migration.  Existence  of 
the  bitter  racial  tensions  that  have  developed  between  white  and  Negro  in  South 
Africa  is  probably  familiar  to  readers.  Less  well  known,  perhaps,  is  the  fact  that 


368  •  EVOLUTION   AND    MAN 

South  Africa  also  has  a  fairly  large  and  rapidly  growing  population  of  immi- 
grants from  India.  This  emigration  has  had  no  noticeable  effect  on  the  rate  of 
growth  of  the  Indian  population.  However,  the  migrants  took  their  low  living 
standards  and  high  birth  rates  with  them  to  South  Africa,  thereby  arousing  the 
resentment  of  both  black  and  white,  and  the  troubles  of  South  Africa  are  now 
being  compounded  by  a  three-way  racial  tension.  In  a  sense  the  problem  has  been 
transplanted  rather  than  solved. 

The  Regulation  of  Man's  Increasing  Numbers 

The  final  possible  solution  to  the  problems  of  the  three-fifths  of  the 
people  who  live  in  either  the  first  or  the  second  stages  of  the  demographic  transi- 
tion is  to  reduce  the  birth  rate  in  step  with  the  reduction  in  the  death  rate  so 
that  numbers  remain  stabilized.  This  solution  seems  to  be  the  one  with  the  great- 
est chance  of  success,  yet  it  is  by  far  the  most  difficult  to  put  into  effect. 

The  situation  in  these  areas  is  distinctly  different  from  that  in  Europe 
two  centuries  ago.  Death  rates  not  only  can  be  but  have  been  brought  down 
drastically  in  a  very  short  period  by  the  application  of  modern  scientific  knowl- 
edge in  backward  areas,  and  the  decrease  has  been  much  more  rapid  than  it  ever 
was  in  Western  Europe.  Consequently,  the  potential  explosive  increase  in  popu- 
lation size  that  exists  in  these  areas  is  far  beyond  what  ever  occurred  in  Europe. 
It  is  also  possible  to  increase  production  in  agriculture  and  industry  in  these 
areas,  although  more  time  and  effort  are  required  than  is  needed  to  reduce  the 
death  rate.  However,  a  reduction  in  the  birth  rate  takes  much  longer  and  is  much 
more  difficult  to  achieve  than  is  the  control  of  deaths  or  an  increase  in  produc- 
tivity. In  the  past,  birth  rates  have  started  to  fall  only  after  standards  of  living 
have  been  raised.  The  highest  birth  rates  are  associated  throughout  the  world 
with  high  levels  of  poverty  and  ignorance.  If  the  historical  sequence  of  events 
is  followed  in  the  underdeveloped  countries  today,  the  outcome  would  appear  to 
be  different  from  that  in  the  Western  nations.  The  reproductive  potential  is  so 
great  that  population  increases,  before  they  can  raise  living  standards  to  the  point 
where  birth  rates  might  be  expected  to  decline,  will  absorb  any  increase  in  pro- 
duction. As  a  consequence,  more  and  more  people  will  be  supported  at  a  bare 
subsistence  level.  The  contrast  between  the  nations  that  have  made  the  demo- 
graphic transition  and  those  that  have  not  will  become  even  more  stark,  and  the 
explosive  possibilities  of  such  a  situation  on  the  international  scene  can  hardly 
be  minimized.  The  conclusion  seems  inescapable,  therefore,  that  countries  today 
that  have  high  birth  and  death  rates  and  that  wish  to  better  the  lot  of  their 
people  and  their  positions  as  nations  must  direct  their  efforts  toward  bringing 
birth  rates  under  control. 

Only  if  the  population  growth  can  be  held  down  can  increased  produc- 
tion be  used  to  improve  living  conditions.  In  the  absence  of  checks  on  growth, 


MAN   AS   A   DOMINANT   SPECIES  •  369 

natural  increase  rather  than  living  standards  responds  to  economic  development. 
The  mere  development  of  underdeveloped  countries  has  never  been  shown 
capable  in  itself  of  raising  living  standards.  Self-generated  development  is  usu- 
ally slow  because  of  the  difficulty  in  amassing  sufficient  capital  and  resources  to 
speed  the  process.  As  a  result,  the  population  increase  rapidly  absorbs  the  gains 
as  they  are  made,  and  as  the  population  grows,  the  problem  of  making  the  demo- 
graphic transition  becomes  increasingly  difficult.  External  aid  on  a  massive  scale 
has  been  suggested  as  a  possible  solution.  However,  outside  aid,  whether  in  the 
form  of  capital,  equipment,  or  technical  aid  or  training,  is  equally  unlikely  to  be 
effective  if  unaccompanied  by  some  means  of  limiting  the  increase  in  population. 
The  experience  of  the  British  in  India  and  Egypt  and  of  the  United  States  in 
Puerto  Rico  point  up  some  of  the  problems  involved.  In  the  decades  of  rule  by 
the  British  in  both  India  and  Egypt,  during  which  the  gross  national  product  of 
the  countries  undoubtedly  increased,  population  growth  more  than  kept  pace 
so  that  today  living  standards  in  these  countries  are  probably  lower  than  they 
were  50  or  more  years  ago.  The  United  States  has  poured  over  a  billion  dollars 
in  aid  into  Puerto  Rico  since  assuming  control  in  1898 — the  greatest  effort  ever 
made  to  put  a  backward  nation  on  its  feet  through  outside  assistance.  The  most 
obvious  result  of  this  aid  has  been  an  increase  in  population  from  about  a  million 
to  more  than  two  and  a  quarter  million.  The  death  rate  per  1000  declined  gradu- 
ally from  31.4  in  1899  to  below  10  per  1000  in  recent  years  while  the  birth  rate, 
which  was  over  40  per  1000  in  1899,  remained  high  until  about  1947  when  a 
slow  decline  set  in.  The  actual  natural  increase  is  still  about  60,000  per  year. 
Emigration  to  the  United  States  has  served  as  a  safety  valve,  for  in  recent  years 
annual  net  emigration  has  almost  equaled  the  natural  increase,  thus  stabilizing 
the  population  size.  Some  progress  toward  raising  the  standard  of  living  has 
been  made  since  about  1945.  However,  unemployment  is  still  common,  and 
housing  and  schools  are  still  inadequate.  Thus  after  60  years  of  generous  aid, 
limited  results  are  finally  forthcoming,  but  Puerto  Rico  has  occupied  such  a 
uniquely  favorable  position  that  the  picture  can  hardly  be  considered  encouraging 
with  respect  to  what  might  be  done  for  other  less  well-situated  areas.  Only  after 
40  years  did  signs  of  progress  appear,  and  Puerto  Rico's  problems  are  by  no 
means  solved  yet.  What  then  can  the  prospects  be  for  the  much  larger  under- 
developed nations  that  can  find  no  place  to  export  their  surplus  population  and 
cannot  hope  to  receive  outside  aid  on  the  same  scale  as  was  used  in  Puerto  Rico  ? 
The  answer  clearly  is  that  the  primary  task  in  the  development  of  the 
have-not  nations  of  the  world  is  the  reduction  of  the  birth  rate  along  with  the 
death  rate  so  that  population  explosions  are  not  detonated  across  the  surface  of 
the  earth.  Reduction  in  the  birth  rate  must  accompany  the  agricultural,  industrial, 
and  medical  revolutions,  and  not  lag  behind.  The  pattern  of  the  past  will  some- 
how have  to  be  broken.  To  do  so  will  not  be  easy,  for  it  represents  a  major  effort 
in  educating  peoples  who  are  illiterate,  poverty  stricken,  and  hunger  ridden,  and 


370  •  EVOLUTION   AND   MAN 

usually  not  particularly  interested  in  this  type  of  education.  The  task  may  be 
further  hampered  by  religious,  ethical,  or  moral  scruples  and  by  legal  or  political 
barriers  to  the  dissemination  of  such  information.  It  may  involve  educating  not 
only  the  common  people  but  their  leaders,  for  before  the  solution  can  be  at- 
tempted, the  problem  itself  must  be  clearly  recognized  and  generally  understood. 
Since  fertility  has  customarily  been  admired  in  most  societies  in  the  past,  a  major 
shift  in  attitude  will  be  required  of  many  peoples.  The  freedom  to  have  children 
must  certainly  be  ranked  with  the  Four  Freedoms  or  any  other  of  the  basic 
human  rights.  In  fact,  it  might  well  be  argued  that  the  right  to  reproduce  is  the 
most  fundamental  of  all  human  rights.  Therefore,  any  program  designed  to  re- 
duce the  birth  rate  must,  if  it  is  to  be  in  accord  with  democratic  principles,  some- 
how be  based  on  the  voluntary  cooperation  of  each  couple  rather  than  enforced 
by  decree. 

The  Roman  Catholic  Church  is  often  pictured  as  being  opposed  to  con- 
trol of  the  birth  rate;  this  is,  in  fact,  not  so,  for  the  Church  approves  of  such 
control  in  principle  but  is  opposed  to  certain  of  the  methods,  whch  are  consid- 
ered "unnatural."  It  is  to  be  hoped  that  other  religions  and  other  cultures  will 
also  approve  in  principle  and  that  effective  methods  for  control  will  be  found 
that  are  acceptable  to  the  great  majority  of  the  peoples  of  the  world.  Much 
research  still  needs  to  be  done  in  this  area,  but  present  results  indicate  that 
simple,  inexpensive,  and  effective  methods  may  soon  be  available. 

Lest  those  nations  not  now  troubled  by  overpopulation  or  likely  to  be  in 
the  foreseeable  future  stand  aside  and  regard  the  problem  as  not  being  a  matter 
of  concern  to  them,  the  genesis  of  World  War  II  should  be  recalled.  In  essence, 
three  nations,  each  nearing  completion  of  the  demographic  transition,  attempted 
to  relieve  their  growing  population  pressure  by  expansion.  Germany  sought 
Lebensraum  to  the  east  in  Poland  and  the  Ukraine,  Italy  expanded  into  North 
Africa,  while  Japan  overran  China  and  many  of  the  Pacific  islands.  The  in- 
stability and  dissatisfaction  generated  in  overpopulated  areas  will  continue  to  be 
a  threat  to  world  peace,  for  human  dignity,  human  rights,  and  human  life  have 
little  value  or  meaning  in  these  areas.  Therefore,  overpopulated  areas  should  be 
a  matter  of  concern  to  all,  and  steps  must  be  taken  to  raise  living  standards 
through  agricultural  and  economic  development.  However,  unless  population 
increase  is  controlled,  all  such  efforts  seem  destined  to  failure.  The  most  hopeful 
development  in  recent  years  is  that  the  governments  of  Japan  and  India,  two 
nations  beset  by  the  problems  of  more  people  than  resources  with  which  to 
support  them,  have  officially  recognized  the  problem  and  have  taken  steps  to  aid 
their  people  in  limiting  the  size  of  their  families.  The  experience  gained  in  these 
countries  and  their  degree  of  success  will  be  of  great  interest  and  significance  to 
the  rest  of  the  world  in  its  search  for  a  better  and  a  happier  life  for  all  mankind. 

Another  solution  to  the  problem  of  overpopulation  is  suggested  in  a 
passage  written  by  Hendrik  Willem  van  Loon  nearly  thirty  years  ago. 


MAN    AS   A    DOMINANT    SPECIES  •   371 


Fig.  35-3.     One  possible  solution  to  the  population  problem.  (With 
permission  from  Van  Loon's  Geography.) 


372  •  EVOLUTION   AND   MAN 

It  sounds  incredible,  but  nevertheless  it  is  true.  If  everybody  in  this  world 
of  ours  were  six  feet  tall  and  a  foot  and  a  half  wide  and  a  foot  thick  (and  that  is 
making  people  a  little  bigger  than  they  usually  are),  then  the  whole  of  the  human 
race  (and  according  to  the  latest  available  statistics  there  are  now  nearly 
2,000,000,000  descendants  of  the  original  Homo  sapiens  and  his  wife)  could  be 
packed  into  a  box  measuring  half  a  mile  in  each  direction.  That,  as  I  just  said, 
sounds  incredible,  but  if  you  don't  believe  me,  figure  it  out  for  yourself  and  you 
will  find  it  to  be  correct. 

If  we  transported  that  box  to  the  Grand  Canyon  of  Arizona  and  balanced 
it  neatly  on  the  low  stone  wall  that  keeps  people  from  breaking  their  necks  when 
stunned  by  the  incredible  beauty  of  that  silent  witness  of  the  forces  of  Eternity,  and 
then  called  little  Noodle,  the  dachschund,  and  told  him  (the  tiny  beast  is  very  intel- 
ligent and  loves  to  oblige)  to  give  the  unwieldy  contraption  a  slight  push  with  his 
soft  brown  nose,  there  would  be  a  moment  of  crunching  and  ripping  as  the  wooden 
planks  loosened  stones  and  shrubs  and  trees  on  their  downward  path,  and  then  a 
low  and  even  softer  bumpity-bumpity-bump  and  a  sudden  splash  when  the  outer 
edges  struck  the  banks  of  the  Colorado  River. 

Then  silence  and  oblivion. 

The  human  sardines  in  their  mortuary  chest  would  soon  be  forgotten. 

The  Canyon  would  go  on  battling  wind  and  air  and  sun  and  rain  as  it  has 
done  since  it  was  created. 

The  world  would  continue  to  run  its  even  course  through  the  uncharted 
heavens. 

The  astronomers  on  distant  and  nearby  planets  would  have  noticed  noth- 
ing out  of  the  ordinary. 

A  century  from  now,  a  little  mound,  densely  covered  with  vegetable 
matter,  would  perhaps  indicate  where  humanity  lay  buried. 

And  that  would  be  all. 

Let  us  hope  that  it  never  comes  to  this.  However,  if  perchance  one 
starry-eyed  young  couple  were  somehow  overlooked  and  if  they  then  doubled 
their  numbers  every  25  years  for  just  32  generations,  in  800  years  they  would 
have  over  4  billion  living  descendants.  Such,  as  Malthus  might  say,  is  the  power 
of  population. 

SUGGESTED  READING 

Darwin,  C.  G.,  I960.  "Can  man  control  his  numbers?"  Evolution  after  Darwin, 
Vol.  2,  The  evolution  of  man,  Sol  Tax,  ed.  Chicago:  University  of  Chicago 
Press. 

Malthus,  T.  R.,  1798.  Essay  on  population,  1st  ed.  Ann  Arbor  Paperbacks  (1959). 
4th  ed.,   1807. 

Population  bulletin.  Washington,  D.  C:  Population  Reference  Bureau. 

Van  Loon,  H.  W.,  1932.  Van  Loon's  Geography.  New  York:  Simon  and  Schuster. 

World  population  and  resources,  1955.  Fairlawn,  N.  J.:  Essential  Books. 


CHAPTER 


36 

Man's  Future 


Predictions  are  so  often  wrong,  even  about  such  relatively 
simple  matters  as  horse  races  or  football  games,  that  the  effort  to 
make  them  hardly  seems  worthwhile.  However,  forecasts  continue 
to  be  made,  perhaps  for  the  prognosticator's  occasional  satisfac- 
tion in  being  right,  more  probably  as  a  guide  in  determining  a 
course  of  action.  Since  the  question  of  man's  future  is  extremely 
complex,  anyone  embarking  on  this  sort  of  crystal-gazing  expedi- 
tion should  go  well  equipped  with  a  supply  of  conditional  clauses. 


Man's  Future  as  a  Species 

One  basis  for  predicting  the  future  is  to  examine  the 
past.  The  first  conclusion  to  be  drawn  from  the  past  is  that  more 
than  99  percent  of  all  animal  species  have  become  extinct.  Some 
of  them  disappeared  in  the  process  of  evolving  into  something 
different,  but  most  of  them  came  to  a  complete  dead  end;  extinc- 
tion was  final  and  irrevocable.  Since  there  is  really  no  reason  to 
suppose  that  man  has  a  tighter  grip  on  immortality  than  any  other 
species,  the  chances  seem  quite  good  that  the  ultimate  fate  of 
Homo  sapiens,  like  that  of  Neanderthal  man,  will  be  extinction. 
After  all,  men  like  ourselves  did  not  become  common  on  the  face 
of  the  earth  until  less  than  50,000  years  ago,  a  mere  drop  in  the 
bucket  of  time. 

From  quite  another  point  of  view,  the  evolutionary  line 
that  has  given  rise  to  man  has  persisted  for  millions  and  millions 
of  years,  and  it  might  therefore  be  expected,  on  the  basis  of  its 


373 


374  •  EVOLUTION   AND    MAN 

previous  success,  to  persist  a  while  longer.  In  this  event,  however,  in  view 
of  the  rapid  rate  of  evolution  in  the  Hominidae  during  the  past  million 
years,  Homo  sapiens  can  be  expected  to  continue  to  evolve,  eventually 
into  an  hominid  population  sufficiently  different  from  Homo  sapiens  to 
be  recognized  as  a  new  species.  In  either  case,  man  as  we  know  him  today 
seems  unlikely  to  persist  indefinitely.  This  you  may  regard  as  fortunate  or  un- 
fortunate, depending  upon  your  point  of  view.  Although  we  may  prefer  to  think 
that  man  in  some  form  will  continue  to  exist,  the  realization  that  we  are  not 
immune  from  complete  extinction  may  lead  eventually  to  a  greater  maturity  in 
political  and  social  thought  than  is  generally  in  evidence  now. 

As  we  discussed  earlier,  the  human  beings  now  living  on  the  earth  form 
a  single  polymorphic,  polytypic  species,  Homo  sapiens.  The  advent  of  more 
efficient  transportation  and  the  resulting  greater  ease  of  movement  and  contact 
among  human  groups  have  led  to  a  breakdown  in  genetic  isolates  and  an  increase 
in  gene  flow  among  different  human  populations.  Although  this  tendency  has 
not  resulted  in  the  obliteration  of  racial  differences,  there  can  be  little  question 
that  hybridization  is  a  greater  factor  in  human  evolution  now  than  at  any  time 
in  the  past.  Furthermore,  this  situation  seems  likely  to  continue. 

Man's  Future  Numbers 

Another  fairly  safe  prediction  is  that  the  human  population  will  con- 
tinue to  increase  in  numbers  in  the  near  future.  Even  safer  is  the  prediction  that 
this  increase  in  population  jize  cannot  continue  unchecked  indefinitely.  Sooner 
or  later  death  rates  will  equal  birth  rates,  and  population  growth  will  cease.  The 
significant  question  is  whether  the  death  rates  will  rise  to  match  high  birth  rates, 
which  would  signalize  a  painful,  tragic  decline  in  standards  of  living,  or  whether 
they  will  equilibrate  at  a  low  level.  Birth  and  death  rates  may  seem  to  be  crude 
indices  of  civilization,  culture,  or  standards  of  living;  nevertheless,  they  are  at 
present  very  sensitive  indicators  of  the  status  of  a  society.  Man's  future  to  a  large 
extent  will  depend  upon  how  successfully  the  human  population  adjusts  to  the 
available  resources.  Very  few  people  accept  a  bare  subsistence  level  as  an  ade- 
quate way  of  life,  but  if  population  expansion  continues,  this  is  the  status  that 
all  mankind  will  eventually  reach.  Before  they  do,  however,  bitter  and  deva- 
stating conflicts  seem  inevitable.  Since  human  population  growth  has  been  due  to 
the  dramatic  reduction  in  the  death  rate,  it  is  clear  that  generally  acceptable 
means  of  controlling  birth  rates  are  essential  if  the  population  explosion  is  to  be 
controlled  before  it  leads  to  disaster. 

Homo  sapiens  is  a  dominant  species  because  of  the  superior  intelligence 
of  its  members.  This  mental  ability  made  possible  the  development  of  culture; 
and  cultural  evolution,  as  distinct  from  biological  evolution,  has  added  a  new 
dimension  to  the  process  of  evolutionary  change.  It  seems  safe  to  predict  that 


man's  future  •  375 

cultural  progress  will  continue.  One  need  only  mention  progress  since  the  turn 
of  the  century  in  such  fields  as  physics,  aeronautics,  genetics,  and  medicine,  to 
emphasize  what  tremendous  strides  have  been  made.  The  end  to  this  advance  is 
not  yet  in  sight.  However,  cultural  evolution  has  not  superceded  biological  evo- 
lution but  has  supplemented  it.  Biological  evolution  will  continue  in  man,  under 
the  influence  of  the  same  evolutionary  forces  that  have  affected  man  as  well  as 
other  species  in  the  past.  Modern  medical  discoveries  have  not  eliminated  the 
operation  of  selection  in  human  populations;  rather,  the  selection  pressures  have 
been  modified  or  changed.  The  factors  affecting  reproductive  fitness  in  modern 
society  may  be  different  from  those  operating  in  a  primitive  society,  but  there  is 
no  reason  to  suppose  that  selection  has  ceased  to  function  altogether. 

Man's  Genetic  Future 

It  seems  probable  that  the  human  "load  of  mutations,"  the  frequency  of 
deleterious  genes  in  the  human  population,  will  continue  to  increase  in  the  near 
future.  Because  of  their  effects  on  mutation  rates,  the  advent  of  the  atomic  age 
and  the  widespread  use  of  mutagenic  ionizing  radiations  in  industry  and  in 
medical  practice  will  be  responsible  in  part  for  this  increase.  To  the  extent  that 
medicine  is  successful  in  counteracting  the  harmful  effects  of  deleterious  genes 
so  that  affected  individuals  survive  and  reproduce,  the  frequencies  of  such  genes 
will  increase.  It  is  not  yet  possible  to  predict  just  how  serious  the  effects  of  these 
trends  may  be,  but  it  hardly  seems  likely  that  they  will  be  favorable.  Rather, 
there  will  be  a  somewhat  greater  percentage  of  persons  who  by  medical  or  other 
environmental  manipulations  must  counteract  the  harmful  effects  of  their  genes. 

The  question  has  been  raised  as  to  whether  current  trends  are  not  lead- 
ing to  a  dissipation  of  the  favorable  genotypes  of  the  past  and  to  an  increase  in 
the  frequency  of  deleterious  or  unfavorable  genes  in  human  populations.  This 
question  is  a  very  fundamental  one,  for  even  though  cultural  or  environmental 
remedies  can  to  some  extent  compensate  for  genetic  deficiencies,  there  must  be  a 
point  beyond  which  such  measures  are  inadequate.  If  too  great  a  proportion  of 
the  population  were  to  pass  that  point,  any  modern  society  would  collapse.  Lest 
you  feel  that  this  picture  is  an  exaggeration,  consider  what  would  happen  if  a 
group  of  chimpanzees  were  made  responsible  for  running  a  large  city.  No  matter 
how  carefully  they  were  trained  for  their  jobs  from  birth  onward,  chaos  would 
result,  for  the  tasks  would  be  beyond  the  capacity  of  their  genotypes  even  if  they 
were  all  exceptionally  able  chimpanzees.  Concern  about  the  possible  genetic 
deterioration  of  man  has  been  expressed  because  so  many  factors  at  present  seem 
to  be  favoring  an  increase  in  frequency  of  harmful  genes  in  human  populations. 
In  addition  to  the  increased  load  of  mutations  mentioned  above,  differential 
fertility  in  many  countries  leads  to  a  disproportionate  number  of  children  being 
born  to  the  parents  least  able  to  give  them  a  favorable  home  environment  and 


376  •  EVOLUTION   AND   MAN 

least  likely  to  endow  them  with  a  favorable  genotype.  In  the  United  States,  for 
example,  one  sixth  of  the  women  are  now  bringing  one  half  of  the  children  of 
the  next  generation  into  families  with  only  one  tenth  of  the  national  income. 
Since  a  laissez-faire  policy  seems  likely  to  lead  to  a  loss  in  genetic  value,  a 
number  of  eugenic  programs  have  been  proposed,  aimed  at  the  genetic  betterment 
of  mankind.  Because  of  the  radical  nature  of  some  of  these  proposals,  especially 
by  early  proponents,  and  because  the  Nazi  pogroms  were  carried  out  under  the 
guise  of  a  eugenics  program,  the  term  "eugenics"  has  come  to  have  rather  sinister 
connotations.  The  current  arguments  for  the  need  for  eugenic  measures  are  based 
on  the  evidence  that  the  net  effect  of  many  human  activities  is  at  present  leading 
to  a  deterioration  of  the  human  gene  pool.  It  is  argued  that  we  cannot  afford  to 
let  this  deterioration  continue  unchecked  but  must  apply  our  present  knowledge 
to  human  genetic  improvement  just  as,  through  conscious  effort,  we  have  im- 
proved domesticated  species  of  plants  and  animals.  Two  types  of  programs  have 
been  suggested:  positive  eugenic  measures  to  increase  the  frequency  of  favorable 
genes  and  gene  combinations,  and  negative  eugenic  measures  to  reduce  the  fre- 
quency of  deleterious  genes.  All  of  these  measures  merit  thoughtful  considera- 
tion, but  they  also  require  careful  scrutiny  because  of  the  risks  inherent  in  any 
program  of  deliberate  interference  with  human  reproduction. 

Eugenics 

The  great  difficulty  with  any  positive  eugenics  program  is  that  decisions 
must  be  made  as  to  which  traits  are  to  be  favored.  These  decisions  will  be  based 
on  value  judgments,  for  they  cannot  be  made  in  any  scientific  manner.  There- 
fore, the  primary  question  becomes,  whose  set  of  values  shall  prevail,  for  it  is 
unlikely  that  there  would  be  any  universal  agreement  sufficiently  specific  to  per- 
mit setting  up  an  effective  program.  Any  program  put  into  effect  without  uni- 
versal acceptance  would  represent  an  unwarranted  infringement  on  human  rights. 
Furthermore,  it  may  even  be  an  error  to  assume  that  human  evolution  should  be 
guided  toward  any  single  goal  or  set  of  values.  The  genetic  problems  involved 
in  breeding  a  new  type  of  corn  or  hog  are  relatively  simple.  The  measure  of 
success  is  in  the  increased  economic  value  of  the  product,  but  this  is  not  the  way 
we  measure  men. 

At  present,  negative  eugenics  seems  more  likely  to  be  accepted  because 
it  is  generally  agreed  that  traits  such  as  hereditary  blindness,  deafness,  or  similar 
severe  afflictions  are  undesirable.  For  this  reason  it  is  possible  through  genetic 
counseling  to  convey  to  the  persons  concerned  sufficient  understanding  of  the 
hereditary  risks  involved  so  that  they  can  make  informed  decisions  concerning 
their  own  reproduction.  Institutionalization  of  mentally  defective  or  psychotic 
persons  is  a  eugenic  measure,  since  they  do  not  ordinarily  reproduce  while  insti- 
tutionalized. The  usefulness  of  negative  eugenics  has  sometimes  been  questioned 


man's  future  •  377 

on  the  grounds  that  its  effect  in  reducing  the  frequency  of  recessive  genes  is  so 
slight.  However,  from  a  humanitarian  standpoint  any  action  that  averts  the  birth 
of  a  single  afflicted  person  must  be  regarded  as  beneficial. 

The  effectiveness  of  negative  eugenics  could  be  greatly  enhanced  if  we 
had  means  to  detect  heterozygous  carriers  of  deleterious  recessive  genes.  Some 
traits  can  now  be  detected  in  heterozygotes,  and  it  seems  probable  that  as  more 
refined  techniques  are  discovered,  additional  information  of  this  sort  will  become 
available.  A  quick  reduction  in  the  incidence  of  individuals  affected  by  harmful 
dominant  genes  is  already  possible;  detection  of  heterozygous  carriers  would 
make  it  possible  to  reduce  still  further  the  incidence  of  persons  afflicted  with  re- 
cessive hereditary  diseases. 

The  success  of  such  a  program  would  depend  upon  the  voluntary  co- 
operation of  a  well-informed  people  and  would  have  to  be  based  on  the  universal 
desire  of  parents  to  have  normal,  healthy  children.  Any  approach  involving  coer- 
cion could  not  be  justified  in  a  society  that  even  pretended  to  be  free. 

It  may  be  argued  that  a  program  of  such  limited  objectives  is  not  ade- 
quate in  the  face  of  such  threats  to  man's  heritage  as  an  increased  load  of  muta- 
tions or  differential  fertility.  However,  we  know  very  little  about  the  magnitude 
or  even  the  direction  of  the  selection  pressures  operative  in  man  at  the  present 
time.  For  example,  it  is  well  known  that  the  average  life  span  of  married  men  is 
longer  than  that  of  bachelors,  a  statistic  frequently  cited  as  evidence  for  the  bene- 
ficial effects  of  a  life  of  wedded  bliss.  If  one  were  to  weigh  all  of  the  variables 
involved,  one  might  conclude  that  the  bachelors,  rather  than  the  married  men, 
had  every  right  to  expect  a  longer  life  span.  An  alternative  explanation  for  this 
fact  is  that  women  tend  to  marry  the  healthier  men  and  that  a  selective  process 
of  considerable  genetic  significance,  rather  than  an  environmental  effect,  is  re- 
sponsible for  the  difference  in  life  span.  A  careful  study  would  be  necessary  to 
determine  which  of  these  alternatives  is  correct. 

Another  bit  of  data  of  possible  significance  is  the  fact  that  in  the  United 
States,  on  the  average,  only  about  90  percent  of  all  women  past  reproductive  age 
have  ever  married.  Furthermore,  among  such  married  women  about  15  to  20 
percent  have  never  had  any  children.  Thus,  the  total  reproductive  burden  is 
being  carried  by  only  three  quarters  of  the  women  in  any  generation.  There  is 
no  evidence  whatever  that  there  are  any  genetic  differences  between  women  who 
marry  and  those  who  do  not,  or  between  married  women  who  have  children  and 
those  who  do  not.  However,  the  proportions  involved  are  so  great  that  if  any 
genetic  differentials  are  involved,  they  could  be  of  considerable  importance. 
Research  to  test  these  possibilities  has  yet  to  be  carried  out.  Until  these  and  other 
possibilities  for  positive  selection  pressure  have  been  explored,  the  extent  of  the 
genetic  deterioration  of  the  human  gene  pool  cannot  be  estimated  with  any 
degree  of  confidence.  The  great  and  obvious  need  is  for  more  research,  not  just 
in  medical  genetics,  but  in  all  aspects  of  human  genetics. 


378  •  EVOLUTION   AND    MAN 

Where  actions  affecting  human  reproduction  are  already  being  taken,  it 
is  clear  that  some  attention  should  be  paid  to  their  eugenic  implications.  Arti- 
ficial insemination,  for  example,  is  being  done  on  an  ever-wider  scale,  and  here 
the  responsibility  for  serious  consideration  of  the  genotype  of  the  donor  is  clear. 
Furthermore,  persons  with  a  corrected  or  ameliorated  genetic  condition  should 
certainly  be  made  aware  of  the  genetic  risks  involved  in  their  reproduction  and 
of  their  responsibility  to  future  generations.  The  point  is  that  as  other  medical, 
biological,  and  genetic  techniques  are  discovered,  they  will  unquestionably  be 
used,  and  they  will  also  undoubtedly  affect  the  course  of  human  evolution.  The 
problem  is  to  insure  that  these  discoveries  are  used  with  wisdom  and  under- 
standing so  that  man's  genetic  heritage,  certainly  his  most  precious  possession,  is 
not  needlessly  frittered  away. 

So  much  for  man's  future;  what  about  future  man  himself?  If  still 
here,  he  will  probably  be  somewhat  different  from  us  physically.  If  past  trends 
continue,  his  head  may  well  be  larger  than  ours,  with  the  face  and  teeth 
still  further  reduced.  His  personality  may  be  such  that  we  would  consider  him  a 
genius,  or  perhaps  a  dolt,  a  criminal,  or  a  crackpot,  or  even  quite  normal. 
Whether  we  would  like  him  or  not  is  of  little  consequence,  for  we  shall  never 
have  to  try  to  get  along  with  him. 

SUGGESTED  READING 

Haldane,  J.  B.  S.,  1949.  "Human  evolution:  past  and  future,"  Genetics,  paleontology 

and  evolution.  G.  L.  Jepsen,  E.  Mayr,  and  G.  G.  Simpson,  eds.  Princeton, 

N.  J. :  Princeton  University  Press. 
Muller,  H.  J.,  I960.  "The  guidance  of  human  evolution,"  Evolution  after  Darwin, 

Vol.  2,  The  evolution  of  man,  Sol  Tax,  ed.  Chicago:  University  of  Chicago 

Press. 
Osborn,  F.,  1951.  Preface  to  eugenics.  New  York:  Harper. 
Reed,  S.  C,  1955.  Counseling  in  medical  genetics.  Philadelphia:  Saunders. 


APPENDIX 


A 


From  Charles  Darwin's 
Voyage  of  the  Beagle 


APPENDIX 


B 


From  Thomas  Malthus' 
Essay  on  the  Principle 

of  Population  4th  edition 


APPENDIX 


A 


Chapter  XVII— 
Galapagos  Archipelago 


September  15th. — This  archipelago  consists  of  ten  principal  islands,  of 
which  five  exceed  the  others  in  size.  They  are  situated  under  the  Equator,  and 
between  five  and  six  hundred  miles  westward  of  the  coast  of  America.  They  are 
all  formed  of  volcanic  rocks;  a  few  fragments  of  granite  curiously  glazed  and 
altered  by  the  heat,  can  hardly  be  considered  as  an  exception.  Some  of  the 
craters,  surmounting  the  larger  islands,  are  of  immense  size,  and  they  rise  to  a 
height  of  between  three  and  four  thousand  feet.  Their  flanks  are  studded  by 
innumerable  smaller  orifices.  I  scarcely  hesitate  to  affirm,  that  there  must  be  in 
the  whole  archipelago  at  least  two  thousand  craters.  These  consist  either  of  lava 
and  scoriae,  or  of  finely-stratified,  sandstone-like  tuff.  Most  of  the  latter  are  beau- 
tifully symmetrical;  they  owe  their  origin  to  eruptions  of  volcanic  mud  without 
any  lava:  it  is  a  remarkable  circumstance  that  every  one  of  the  twenty-eight  tuff- 
craters  which  were  examined,  had  their  southern  sides  either  much  lower  than 
the  other  sides,  or  quite  broken  down  and  removed.  As  all  these  craters  appar- 
ently have  been  formed  when  standing  in  the  sea,  and  as  the  waves  from  the 
trade  wind  and  the  swell  from  the  open  Pacific  here  unite  their  forces  on  the 
southern  coasts  of  all  the  islands,  this  singular  uniformity  in  the  broken  state  of 
the  craters,  composed  of  the  soft  and  yielding  tuff,  is  easily  explained. 

From  Charles  Darwin,  1887.  Journal  of  researches  into  the  natural  history  and  geology 
of  the  countries  visited  during  the  voyage  of  H.M.S.  Beagle  round  the  world.  New  ed. 
New  York:   D.  Appleton  and  Company.    Pages  372-73,  377-81,  393-98. 

381 


382   •  APPENDIX 

Considering  that  these  islands  are  placed  directly  under  the  equator,  the 
climate  is  far  from  being  excessively  hot;  this  seems  chiefly  caused  by  the  singu- 
larly low  temperature  of  the  surrounding  water,  brought  here  by  the  great 
southern  Polar  current.  Excepting  during  one  short  season,  very  little  rain  falls, 
and  even  then  it  is  irregular;  but  the  clouds  generally  hang  low.  Hence,  whilst 
the  lower  parts  of  the  islands  are  very  sterile,  the  upper  parts,  at  a  height  of  a 
thousand  feet  and  upwards,  possess  a  damp  climate  and  a  tolerably  luxuriant 
vegetation.  This  is  especially  the  case  on  the  windward  sides  of  the  islands,  which 
first  receive  and  condense  the  moisture  from  the  atmosphere.  .  .  . 

The  natural  history  of  these  islands  is  eminently  curious,  and  well  de- 
serves attention.  Most  of  the  organic  productions  are  aboriginal  creations,  found 
nowhere  else;  there  is  even  a  difference  between  the  inhabitants  of  the  different 
islands;  yet  all  show  a  marked  relationship  with  those  of  America,  though  sepa- 
rated from  that  continent  by  an  open  space  of  ocean,  between  500  and  600  miles 
in  width.  The  archipelago  is  a  little  world  within  itself,  or  rather  a  satellite  at- 
tached to  America,  whence  it  has  derived  a  few  stray  colonists,  and  has  received 
the  general  character  of  its  indigenous  productions.  Considering  the  small  size  of 
these  islands,  we  feel  the  more  astonished  at  the  number  of  their  aboriginal 
beings,  and  at  their  confined  range.  Seeing  every  height  crowned  with  its  crater, 
and  the  boundaries  of  most  of  the  Java-streams  still  distinct,  we  are  led  to  believe 
that  within  a  period,  geologically  recent,  the  unbroken  ocean  was  here  spread 
out.  Hence,  both  in  space  and  time,  we  seem  to  be  brought  somewhat  near  to 
that  great  fact — that  mystery  of  mysteries — the  first  appearance  of  new  beings  on 
this  earth. 

Of  terrestrial  mammals,  there  is  only  one  which  must  be  considered  as 
indigenous,  namely,  a  mouse  (Mus  Galapagoensis),  and  this  is  confined,  as  far 
as  I  could  ascertain,  to  Chatham  island,  the  most  easterly  island  of  the  group.  It 
belongs,  as  I  am  informed  by  Mr.  Waterhouse,  to  a  division  of  the  family  of 
mice  characteristic  of  America.  At  James  island,  there  is  a  rat  sufficiently  distinct 
from  the  common  kind  to  have  been  named  and  described  by  Mr.  Waterhouse; 
but  as  it  belongs  to  the  old-world  division  of  the  family,  and  as  this  island  has 
been  frequented  by  ships  for  the  last  hundred  and  fifty  years,  I  can  hardly  doubt 
that  this  rat  is  merely  a  variety,  produced  by  the  new  and  peculiar  climate,  food, 
and  soil,  to  which  it  has  been  subjected.  Although  no  one  has  a  right  to  speculate 
without  distinct  facts,  yet  even  with  respect  to  the  Chatham  island  mouse,  it 
should  be  borne  in  mind,  that  it  may  possibly  be  an  American  species  imported 
here;  for  I  have  seen,  in  a  most  unfrequented  part  of  the  Pampas,  a  native  mouse 
living  in  the  roof  of  a  newly-built  hovel,  and  therefore  its  transportation  in  a 
vessel  is  not  improbable;  analogous  facts  have  been  observed  by  Dr.  Richardson 
in  North  America. 

Of  land-birds  I  obtained  twenty-six  kinds,  all  peculiar  to  the  group  and 
found  nowhere  else,  with  the  exception  of  one  lark-like  finch  from  North 
America  (Dolichonyx  oryzivorus),  which  ranges  on  that  continent  as  far  north 


APPENDIX  •   383 

as  54°,  and  generally  frequents  marshes.  The  other  twenty-five  birds  consist, 
firstly,  of  a  hawk,  curiously  intermediate  in  structure  between  a  Buzzard  and  the 
American  group  of  carrion-feeding  Polybori;  and  with  these  latter  birds  it  agrees 
most  closely  in  every  habit  and  even  tone  of  voice.  Secondly,  there  are  two  owls, 
representing  the  short-eared  and  white  barn-owls  of  Europe.  Thirdly,  a  wren, 
three  tyrant  fly-catchers  (two  of  them  species  of  Pyocephalus,  one  or  both  of 
which  would  be  ranked  by  some  ornithologists  as  only  varieties),  and  a  dove — 
all  analogous  to,  but  distinct  from,  American  species.  Fourthly,  a  swallow,  which 
though  differing  from  the  Progne  purpurea  of  both  Americas,  only  in  being 
rather  duller  coloured,  smaller,  and  slenderer,  is  considered  by  Mr.  Gould  as 
specifically  distinct.  Fifthly,  there  are  three  species  of  mocking-thrush — a  form 
highly  characteristic  of  America.  The  remaining  land-birds  form  a  most  singular 
group  of  finches,  related  to  each  other  in  the  structure  of  their  beaks,  short  tails, 
form  of  body,  and  plumage:  there  are  thirteen  species,  which  Mr.  Gould  has 
divided  into  four  sub-groups.  All  these  species  are  peculiar  to  this  archipelago; 
and  so  is  the  whole  group,  with  the  exception  of  one  species  of  the  sub-group 
Cactornis,  lately  brought  from  Bow  island,  in  the  Low  Archipelago.  Of  Cactornis, 
the  two  species  may  be  often  seen  climbing  about  the  flowers  of  the  great  cactus- 
trees;  but  all  the  other  species  of  this  group  of  finches,  mingled  together  in 
flocks,  feed  on  the  dry  and  sterile  ground  of  the  lower  districts.  The  males  of  all, 
or  certainly  of  the  greater  number,  are  jet  black;  and  the  females  (with  perhaps 
one  or  two  exceptions)  are  brown.  The  most  curious  fact  is  the  perfect  gradation 
in  the  size  of  the  beaks  in  the  different  species  of  Geospiza,  from  one  as  large  as 
that  of  a  hawfinch  to  that  of  a  chaffinch,  and  (if  Mr.  Gould  is  right  in  including 
his  sub-group,  Certhidea.  in  the  main  group),  even  to  that  of  a  warbler.  The 
largest  beak  in  the  genus  Geospiza  is  shown  in  Fig.  1,  and  the  smallest  in  Fig.  3; 
but  instead  of  there  being  only  one  intermediate  species,  with  a  beak  of  the  size 
shown  in  Fig.  2,  there  are  no  less  than  six  species  with  insensibly  graduated 
beaks.  The  beak  of  the  sub-group  Certhidea,  is  shown  in  Fig.  4.  [Refer  to 
text  Fig.  31-4.]  The  beak  of  Cactornis  is  somewhat  like  that  of  a  starling;  and 
that  of  the  fourth  sub-group,  Camarhynchus,  is  slightly  parrot-shaped.  Seeing 
this  gradation  and  diversity  of  structure  in  one  small,  intimately  related  group 
of  birds,  one  might  really  fancy  that  from  an  original  paucity  of  birds  in  this 
archipelago,  one  species  had  been  taken  and  modified  for  different  ends.  In  a 
like  manner  it  might  be  fancied  that  a  bird  originally  a  buzzard,  had  been  in- 
duced here  to  undertake  the  office  of  the  carrion-feeding  Polybori  of  the  Amer- 
ican continent. 

Of  waders  and  water-birds  I  was  able  to  get  only  eleven  kinds,  and  of 
these  only  three  (including  a  rail  confined  to  the  damp  summits  of  the  islands) 
are  new  species.  Considering  the  wandering  habits  of  the  gulls,  I  was  surprised  to 
find  that  the  species  inhabiting  these  islands  is  peculiar,  but  allied  to  one  from 
the  southern  parts  of  South  America.  The  far  greater  peculiarity  of  the  land- 
birds,  namely,  twenty-five  out  of  twenty-six  being  new  species  or  at  least  new 


384  •  APPENDIX 

races,  compared  with  the  waders  and  web-footed  birds,  is  in  accordance  with  the 
greater  range  which  these  latter  orders  have  in  all  parts  of  the  world.  We  shall 
hereafter  see  this  law  of  aquatic  forms,  whether  marine  or  fresh-water,  being 
less  peculiar  at  any  given  point  of  the  earth's  surface  than  the  terrestrial  forms 
of  the  same  classes,  strikingly  illustrated  in  the  shells,  and  in  a  lesser  degree  in 
the  insects  of  this  archipelago. 

Two  of  the  waders  are  rather  smaller  than  the  same  species  brought 
from  other  places:  the  swallow  is  also  smaller,  though  it  is  doubtful  whether  or 
not  it  is  distinct  from  its  analogue.  The  two  owls,  the  two  tyrant  fly-catchers 
(Pyrocephalus)  and  the  dove,  are  also  smaller  than  the  analogous  but  distinct 
species,  to  which  they  are  most  nearly  related;  on  the  other  hand,  the  gull  is 
rather  larger.  The  two  owls,  the  swallow,  all  three  species  of  mocking-thrush, 
the  dove  in  its  separate  colours  though  not  in  its  whole  plumage,  the  Totanus, 
and  the  gull,  are  likewise  duskier  coloured  than  their  analogous  species;  and  in 
the  case  of  the  mocking-thrush  and  Totanus,  than  any  other  species  of  the  two 
genera.  With  the  exception  of  a  wren  with  a  fine  yellow  breast,  and  of  a  tyrant 
fly-catcher  with  a  scarlet  tuft  and  breast,  none  of  the  birds  are  brilliantly 
coloured,  as  might  have  been  expected  in  an  equatorial  district.  Hence  it  would 
appear  probable,  that  the  same  causes  which  here  make  the  immigrants  of  some 
species  smaller,  make  most  of  the  peculiar  Galapageian  species  also  smaller,  as 
well  as  very  generally  more  dusky  coloured.  All  the  plants  have  a  wretched, 
weedy  appearance,  and  I  did  not  see  one  beautiful  flower.  The  insects,  again,  are 
small  sized  and  dull  coloured,  and,  as  Mr.  Waterhouse  informs  me,  there  is 
nothing  in  their  general  appearance  which  would  have  led  him  to  imagine  that 
they  had  come  from  under  the  equator.  The  birds,  plants,  and  insects  have  a 
desert  character,  and  are  not  more  brilliantly  coloured  than  those  from  southern 
Patagonia;  we  may,  therefore,  conclude  that  the  usual  gaudy  colouring  of  the 
intertropical  productions,  is  not  related  either  to  the  heat  or  light  of  those  zones, 
but  to  some  other  cause,  perhaps  to  the  conditions  of  existence  being  generally 
favourable  to  life. 

.  .  .  Dr.  Hooker  informs  me  that  the  Flora  has  an  undoubted  Western 
American  character;  nor  can  he  detect  in  it  any  affinity  with  that  of  the  Pacific. 
If,  therefore,  we  except  the  eighteen  marine,  the  one  fresh-water,  and  one  land- 
shell,  which  have  apparently  come  here  as  colonists  from  the  central  islands  of 
the  Pacific,  and  likewise  the  one  distinct  Pacific  species  of  the  Galapageian  group 
of  finches,  we  see  that  this  archipelago,  though  standing  in  the  Pacific  Ocean,  is 
zoologically  part  of  America. 

If  this  character  were  owing  merely  to  immigrants  from  America,  there 
would  be  little  remarkable  in  it;  but  we  see  that  a  vast  majority  of  all  the  land 
animals,  and  that  more  than  half  of  the  flowering  plants,  are  aboriginal  produc- 
tions. It  was  most  striking  to  be  surrounded  by  new  birds,  new  reptiles,  new 
shells,  new  insects,  new  plants,  and  yet  by  innumerable  trifling  details  of  struc- 


APPENDIX  •  385 

ture,  and  even  by  the  tones  of  voice  and  plumage  of  the  birds,  to  have  the 
temperate  plains  of  Patagonia,  or  the  hot  dry  deserts  of  Northern  Chile,  vividly 
brought  before  my  eyes.  Why,  on  these  small  points  of  land,  which  within  a  late 
geological  period  must  have  been  covered  by  the  ocean,  which  are  formed  of 
basaltic  lava,  and  therefore  differ  in  geological  character  from  the  American 
continent,  and  which  are  placed  under  a  peculiar  climate, — why  were  their 
aboriginal  inhabitants,  associated,  I  may  add,  in  different  proportions  both  in 
kind  and  number  from  those  on  the  continent,  and  therefore  acting  on  each 
other  in  a  different  manner — why  were  they  created  on  American  types  of 
organization?  It  is  probable  that  the  islands  of  the  Cape  de  Verd  group  resem- 
ble, in  all  their  physical  conditions,  far  more  closely  the  Galapagos  Islands  than 
these  latter  physically  resemble  the  coast  of  America;  yet  the  aboriginal  inhab- 
itants of  the  two  groups  are  totally  unlike;  those  of  the  Cape  de  Verd  Islands 
bearing  the  impress  of  Africa,  as  the  inhabitants  of  the  Galapagos  Archipelago 
are  stamped  with  that  of  America. 

I  have  not  as  yet  noticed  by  far  the  most  remarkable  feature  in  the 
natural  history  of  this  archipelago;  it  is,  that  the  different  islands  to  a  consider- 
able extent  are  inhabited  by  a  different  set  of  beings.  My  attention  was  first  called 
to  this  fact  by  the  Vice-Governor,  Mr.  Lawson,  declaring  that  the  tortoises  dif- 
fered from  the  different  islands,  and  that  he  could  with  certainty  tell  from  which 
island  any  one  was  brought.  I  did  not  for  some  time  pay  sufficient  attention  to 
this  statement,  and  I  had  already  partially  mingled  together  the  collections  from 
two  of  the  islands.  I  never  dreamed  that  islands,  about  fifty  or  sixty  miles  apart, 
and  most  of  them  in  sight  of  each  other,  formed  of  precisely  the  same  rocks, 
placed  under  a  quite  similar  climate,  rising  to  a  nearly  equal  height,  would  have 
been  differently  tenanted;  but  we  shall  soon  see  that  this  is  the  case.  It  is  the  fate 
of  most  voyagers,  no  sooner  to  discover  what  is  most  interesting  in  any  locality, 
than  they  are  hurried  from  it;  but  I  ought,  perhaps,  to  be  thankful  that  I  ob- 
tained sufficient  materials  to  establish  this  most  remarkable  fact  in  the  distribu- 
tion of  organic  beings. 

The  inhabitants,  as  I  have  said,  state  that  they  can  distinguish  the  tor- 
toises from  the  different  islands;  and  that  they  differ  not  only  in  size  but  in 
other  characters.  Captain  Porter  has  described  those  from  Charles  and  from  the 
nearest  island  to  it,  namely,  Hood  Island,  as  having  their  shells  in  front  thick 
and  turned  up  like  a  Spanish  saddle,  whilst  the  tortoises  from  James  Island  are 
rounder,  blacker,  and  have  a  better  taste  when  cooked.  M.  Bibron,  moreover, 
informs  me  that  he  has  seen  what  he  considers  two  distinct  species  of  tortoise 
from  the  Galapagos,  but  he  does  not  know  from  which  islands.  The  specimens 
that  I  brought  from  three  islands  were  young  ones;  and  probably  owing  to  this 
cause,  neither  Mr.  Gray  nor  myself  could  find  in  them  any  specific  differences. 
I  have  remarked  that  the  marine  Amblyrhynchus  was  larger  at  Albemarle  Island 
than  elsewhere;  and  M.  Bibron  informs  me  that  he  has  seen  two  distinct  aquatic 
species  of  this  genus;  so  that  the  different  islands  probably  have  their  representa- 


386  •  APPENDIX 

tive  species  or  races  of  the  Amblyrhynchus,  as  well  as  of  the  tortoise.  My 
attention  was  first  thoroughly  aroused,  by  comparing  together  the  numerous  spe- 
cimens, shot  by  myself  and  several  other  parties  on  board,  of  the  mocking- 
thrushes,  when,  to  my  astonishment,  I  discovered  that  all  those  from  Charles 
Island  belonged  to  one  species  (Mimus  trifasciatus) ;  all  from  Albemarle  Island 
to  M.  parvulus;  and  all  from  James  and  Chatham  Islands  (between  which  two 
other  islands  are  situated,  as  connecting  links)  belonged  to  M.  melanotis.  These 
two  latter  species  are  closely  allied,  and  would  by  some  ornithologists  be  con- 
sidered as  only  well-marked  races  or  varieties;  but  the  Mimus  trifasciatus  is  very 
distinct.  Unfortunately  most  of  the  specimens  of  the  finch  tribe  were  mingled 
together;  but  I  have  strong  reasons  to  suspect  that  some  of  the  species  of  the 
sub-group  Geospiza  are  confined  to  separate  islands.  If  the  different  islands  have 
their  representatives  of  Geospiza,  it  may  help  to  explain  the  singularly  large 
number  of  the  species  of  this  sub-group  in  this  one  small  archipelago,  and  as  a 
probable  consequence  of  their  numbers,  the  perfectly  graduated  series  in  the 
size  of  their  beaks.  Two  species  of  the  sub-group  Cactornis,  and  two  of 
Camarhynchus,  were  procured  in  the  archipelago;  and  of  the  numerous  spe- 
cimens of  these  two  sub-groups  shot  by  four  collectors  at  James  Island,  all  were 
found  to  belong  to  one  species  of  each;  whereas  the  numerous  specimens  shot 
either  on  Chatham  or  Charles  Island  (for  the  two  sets  were  mingled  together) 
all  belonged  to  the  two  other  species:  hence  we  may  feel  almost  sure  that  these 
islands  possess  their  representative  species  of  these  two  sub-groups.  In  land- 
shells  this  law  of  distribution  does  not  appear  to  hold  good.  In  my  very  small 
collection  of  insects,  Mr.  Waterhouse  remarks,  that  of  those  which  were  ticketed 
with  their  locality,  not  one  was  common  to  any  two  of  the  islands. 

If  we  now  turn  to  the  Flora,  we  shall  find  the  aboriginal  plants  of  the 
different  islands  wonderfully  different.  I  give  all  the  following  results  on  the 
high  authority  of  my  friend  Dr.  J.  Hooker.  I  may  premise  that  I  indiscriminately 
collected  everything  in  flower  on  the  different  islands,  and  fortunately  kept  my 
collections  separate.  Too  much  confidence,  however,  must  not  be  placed  in  the 
proportional  results,  as  the  small  collections  brought  home  by  some  other  natural- 
ists, though  in  some  respects  confirming  the  results,  plainly  show  that  much  re- 
mains to  be  done  in  the  botany  of  this  group:  the  Leguminosae,  moreover,  have 
as  yet  been  only  approximately  worked  out.  [See  table  on  next  page,  Ed.] 

Hence  we  have  the  truly  wonderful  fact,  that  in  James  Island,  of  the 
thirty-eight  Galapageian  plants,  or  those  found  in  no  other  part  of  the  world, 
thirty  are  exclusively  confined  to  this  one  island;  and  in  Albemarle  Island,  of  the 
twenty-six  aboriginal  Galapageian  plants,  twenty-two  are  confined  to  this  one 
island,  that  is,  only  four  are  at  present  known  to  grow  in  the  other  islands  of  the 
archipelago;  and  so  on,  as  shown  in  the  table  [below],  with  the  plants  from 
Chatham  and  Charles  Islands.  This  fact  will,  perhaps,  be  rendered  even  more 
striking,  by  giving  a  few  illustrations: — thus,  Scalesia,  a  remarkable  arborescent 
genus  of  the  Compositae,  is  confined  to  the  archipelago:  it  has  six  species;  one 


APPENDIX  •   387 


No.  of  Species 

No.  of 

No.  of 

confined  to  the 

Species 

Species 

No. 

Galapagos 

Name 

Total 

found  in 

confined 

confined 

Archipelago, 

of 

No.  of 

other  parts 
of  the 

to  the 

to  the 

but  found  on 

Island 

Species 

Galapagos 

one 

more  than  the 

world 

Archipelago 

Island 

one  Island 

James  Island 

71 

33 

38 

30 

8 

Albemarle  Island 

46 

18 

26 

22 

4 

Chatham  Island 

32 

16 

16 

12 

4 

Charles  Island 

68 

39 
(or  29,  if 

the  probably- 
imported 
plants  be 

subtracted) 

29 

21 

8 

from  Chatham,  one  from  Albemarle,  one  from  Charles  Island,  two  from  James 
Island,  and  the  sixth  from  one  of  the  three  latter  islands,  but  it  is  not  known 
from  which:  not  one  of  these  six  species  grows  on  any  two  islands.  Again, 
Euphorbia,  a  mundane  or  widely  distributed  genus,  has  here  eight  species,  of 
which  seven  are  confined  to  the  archipelago,  and  not  one  found  on  any  two 
islands:  Acalypha  and  Borreria,  both  mundane  genera,  have  respectively  six  and 
seven  species,  none  of  which  have  the  same  species  on  two  islands,  with  the 
exception  of  one  Borreria,  which  does  occur  on  two  islands.  The  species  of  the 
Compositae  are  particularly  local;  and  Dr.  Hooker  has  furnished  me  with  several 
other  most  striking  illustrations  of  the  difference  of  the  species  on  the  different 
islands.  He  remarks  that  this  law  of  distribution  holds  good  both  with  those 
genera  confined  to  the  archipelago,  and  those  distributed  in  other  quarters  of  the 
world:  in  like  manner  we  have  seen  that  the  different  islands  have  their  proper 
species  of  the  mundane  genus  of  tortoise,  and  of  the  widely  distributed  Amer- 
ican genus  of  the  mocking-thrush,  as  well  as  of  two  of  the  Galapageian  sub- 
groups of  finches,  and  almost  certainly  of  the  Galapageian  genus  Amblyrhynchus. 
The  distribution  of  the  tenants  of  this  archipelago  would  not  be  nearly 
so  wonderful,  if,  for  instance,  one  island  had  a  mocking-thrush,  and  a  second 
island  some  other  quite  distinct  genus; — if  one  island  had  its  genus  of  lizard, 
and  a  second  island  another  distinct  genus,  or  none  whatever; — or  if  the  dif- 
ferent islands  were  inhabited,  not  by  representative  species  of  the  same  genera  of 
plants,  but  by  totally  different  genera,  as  does  to  a  certain  extent  hold  good;  for, 
to  give  one  instance,  a  large  berry-bearing  tree  at  James  Island  has  no  representa- 
tive species  in  Charles  Island.  But  it  is  the  circumstance,  that  several  of  the 
islands  possess  their  own  species  of  the  tortoise,  mocking-thrush,  finches,  and 


388  *  APPENDIX 

numerous  plants,  these  species  having  the  same  general  habits,  occupying  analo- 
gous situations,  and  obviously  filling  the  same  place  in  the  natural  economy  of 
this  archipelago,  that  strikes  me  with  wonder.  It  may  be  suspected  that  some  of 
these  representative  species,  at  least  in  the  case  of  the  tortoise  and  of  some  of  the 
birds,  may  hereafter  prove  to  be  only  well-marked  races;  but  this  would  be  of 
equally  great  interest  to  the  philosophical  naturalist.  I  have  said  that  most  of  the 
islands  are  in  sight  of  each  other:  I  may  specify  that  Charles  Island  is  fifty  miles 
from  the  nearest  part  of  Chatham  Island,  and  thirty-three  miles  from  the  nearest 
part  of  Albemarle  Island.  Chatham  Island  is  sixty  miles  from  the  nearest  part  of 
James  Island,  but  there  are  two  intermediate  islands  between  them  which  were 
not  visited  by  me.  James  Island  is  only  ten  miles  from  the  nearest  part  of  Albe- 
marle Island,  but  the  two  points  where  the  collections  were  made  are  thirty-two 
miles  apart.  I  must  repeat,  that  neither  the  nature  of  the  soil,  nor  height  of  the 
land,  nor  the  climate,  nor  the  general  character  of  the  associated  beings,  and 
therefore  their  action  one  on  another,  can  differ  much  in  the  different  islands. 
If  there  be  any  sensible  difference  in  their  climates,  it  must  be  between  the  wind- 
ward group  (namely  Charles  and  Chatham  Islands),  and  that  to  leeward;  but 
there  seems  to  be  no  corresponding  difference  in  the  productions  of  these  two 
halves  of  the  archipelago. 

The  only  light  which  I  can  throw  on  this  remarkable  difference  in  the  in- 
habitants of  the  different  islands,  is,  that  very  strong  currents  of  the  sea  running 
in  a  westerly  and  W.N.W.  direction  must  separate,  as  far  as  transportal  by  the  sea 
is  concerned,  the  southern  islands  from  the  northern  ones;  and  between  these 
northern  islands  a  strong  N.W.  current  was  observed,  which  must  effectually 
separate  James  and  Albemarle  Islands.  As  the  archipelago  is  free  to  a  most  re- 
markable degree  from  gales  of  wind,  neither  the  birds,  insects,  nor  lighter  seeds, 
would  be  blown  from  island  to  island.  And  lastly,  the  profound  depth  of  the 
ocean  between  the  islands,  and  their  apparently  recent  (in  a  geological  sense) 
volcanic  origin,  render  it  highly  unlikely  that  they  were  ever  united;  and  this, 
probably,  is  a  far  more  important  consideration  than  any  other,  with  respect  to 
the  geographical  distribution  of  their  inhabitants.  Reviewing  the  facts  here  given, 
one  is  astonished  at  the  amount  of  creative  force,  if  such  an  expression  may  be 
used,  displayed  on  these  small,  barren,  and  rocky  islands;  and  still  more  so,  at  its 
diverse  yet  analogous  action  on  points  so  near  each  other.  I  have  said  that  the 
Galapagos  Archipelago  might  be  called  a  satellite  attached  to  America,  but  it 
should  rather  be  called  a  group  of  satellites,  physically  similar,  organically  dis- 
tinct, yet  intimately  related  to  each  other,  and  all  related  in  a  marked,  though 
much  lesser  degree,  to  the  great  American  continent. 


APPENDIX 


B 


An  Essay  on  the  Principle 

of  Population,  Book  I 


Of  the  Checks  to  Population  in  the  Less  Civilized 
Parts  of  the  World  and  in  Past  Times 

Chapter  1  —  statement  of  the  subject. 

RATIOS    OF    THE    INCREASE    OF     POPULATION 
AND  FOOD 

In  an  inquiry  concerning  the  improvement  of  society,  the  mode  of  conducting 
the  subject  which  naturally  presents  itself,  is 

1.  To  investigate  the  causes  that  have  hitherto  impeded  the  progress  of 
mankind  towards  happiness;  and 

2.  To  examine  the  probability  of  the  total  or  partial  removal  of  these 
causes  in  future. 

To  enter  fully  into  this  question,  and  to  enumerate  all  the  causes  that 
have  hitherto  influenced  human  improvement,  would  be  much  beyond  the  power 
of  an  individual.  The  principle  object  of  the  present  essay  is  to  examine  the 
effects  of  one  great  cause  intimately  united  with  the  very  nature  of  man;  which, 

From  T.  R.  Malthus,  1807.   An  essay  on  the  principle  of  population.    Fourth  ed.    London: 
J.  Johnson  in  St.  Paul's  Churchyard.  Chapters  1  and  2. 

389 


390  •  APPENDIX 

though  it  has  been  constantly  and  powerfully  operating  since  the  commencement 
of  society,  has  been  little  noticed  by  the  writers  who  have  treated  this  subject. 
The  facts  which  establish  the  existence  of  this  cause  have,  indeed,  been  repeatedly 
stated  and  acknowledged;  but  its  natural  and  necessary  effects  have  been  almost 
totally  overlooked;  though  probably  among  these  effects  may  be  reckoned  a  very 
considerable  portion  of  that  vice  and  misery,  and  of  that  unequal  distribution  of 
the  bounties  of  nature,  which  it  has  been  the  unceasing  object  of  the  enlightened 
philanthropist  in  all  ages  to  correct. 

The  cause  to  which  I  allude,  is  the  constant  tendency  in  all  animated 
life  to  increase  beyond  the  nourishment  prepared  for  it. 

It  is  observed  by  Dr.  Franklin,  that  there  is  no  bound  to  the  prolific 
nature  of  plants  or  animals,  but  what  is  made  by  their  crowding  and  interfering 
with  each  others  means  of  subsistence.  Were  the  face  of  the  earth,  he  says,  vacant 
of  other  plants,  it  might  be  gradually  sowed  and  overspread  with  one  kind  only, 
as  for  instance  with  fennel :  and  were  it  empty  of  other  inhabitants,  it  might  in 
a  few  ages  be  replenished  from  one  nation  only,  as  for  instance  with  Englishmen. 

This  is  incontrovertibly  true.  Through  the  animal  and  vegetable  king- 
doms Nature  has  scattered  the  seeds  of  life  abroad  with  the  most  profuse  and 
liberal  hand;  but  has  been  comparatively  sparing  in  the  room  and  the  nourish- 
ment necessary  to  rear  them.  The  germs  of  existence  contained  in  this  earth,  if 
they  could  freely  develope  themselves,  would  fill  millions  of  worlds  in  the  course 
of  a  few  thousand  years.  Necessity,  that  imperious  all-pervading  law  of  nature, 
restrains  them  within  the  prescribed  bounds.  The  race  of  plants  and  the  race  of 
animals  shrink  under  this  great  restrictive  law;  and  man  cannot  by  any  efforts  of 
reason  escape  from  it. 

In  plants  and  irrational  animals,  the  view  of  the  subject  is  simple. 
They  are  all  impelled  by  a  powerful  instinct  to  the  increase  of  their  species;  and 
this  instinct  is  interrupted  by  no  doubts  about  providing  for  their  offspring. 
Wherever  therefore  there  is  liberty,  the  power  of  increase  is  exerted;  and  the 
superabundant  effects  are  repressed  afterwards  by  want  of  room  and  nourishment. 

The  effects  of  this  check  on  man  are  more  complicated.  Impelled  to  the 
increase  of  his  species  by  an  equally  powerful  instinct,  reason  interrupts  his 
career,  and  asks  him  whether  he  may  not  bring  beings  into  the  world,  for  whom 
he  cannot  provide  the  means  of  support.  If  he  attend  to  this  natural  suggestion, 
the  restriction  too  frequently  produces  vice.  If  he  hear  it  not,  the  human  race 
will  be  constantly  endeavoring  to  increase  beyond  the  means  of  subsistence.  But 
as  by  that  law  of  our  nature  which  makes  food  necessary  to  the  life  of  man, 
population  can  never  actually  increase  beyond  the  lowest  nourishment  capable 
of  supporting  it,  a  strong  check  on  population,  from  the  difficulty  of  acquiring 
food,  must  be  constantly  in  operation.  This  difficulty  must  fall  somewhere,  and 
must  necessarily  be  severely  felt  in  some  or  other  of  the  various  forms  of  misery, 
or  the  fear  of  misery,  by  a  large  portion  of  mankind. 

That  population   has  this  constant  tendency  to  increase  beyond  the 


APPENDIX  •  391 

means  of  subsistence,  and  that  it  is  kept  to  its  necessary  level  by  these  causes, 
will  sufficiently  appear  from  a  review  of  the  different  states  of  society  in  which 
man  has  existed.  But  before  we  proceed  to  this  review,  the  subject  will  perhaps 
be  seen  in  a  clearer  light,  if  we  endeavour  to  ascertain,  what  would  be  the 
natural  increase  of  population,  if  left  to  exert  itself  with  perfect  freedom;  and 
what  might  be  expected  to  be  the  rate  of  increase  in  the  productions  of  the 
earth,  under  the  most  favourable  circumstances  of  human  industry. 

It  will  be  allowed,  that  no  country  has  hitherto  been  known,  where  the 
manners  were  so  pure  and  simple,  and  the  means  of  subsistence  so  abundant, 
that  no  check  whatever  has  existed  to  early  marriages  from  the  difficulty  of  pro- 
viding for  a  family,  and  that  no  waste  of  the  human  species  has  been  occasioned 
by  vicious  customs,  by  towns,  by  unhealthy  occupations,  or  too  severe  labour. 
Consequently  in  no  state  that  we  have  yet  known,  has  the  power  of  population 
been  left  to  exert  itself  with  perfect  freedom. 

Whether  the  law  of  marriage  be  instituted,  or  not,  the  dictate  of  nature 
and  virtue  seems  to  be  an  early  attachment  to  one  woman;  and  where  there  were 
no  impediments  of  any  kind  in  the  way  of  an  union  to  which  such  an  attachment 
would  lead,  and  no  causes  of  depopulation  afterwards,  the  increase  of  the  human 
species  would  be  evidently  much  greater  than  any  increase  which  has  been 
hitherto  known. 

In  the  northern  states  of  America,  where  the  means  of  subsistence  have 
been  more  ample,  the  manners  of  the  people  more  pure,  and  the  checks  to  early 
marriages  fewer,  than  in  any  of  the  modern  states  of  Europe,  the  population  has 
been  found  to  double  itself,  for  above  a  century  and  a  half  successively,  in  less 
than  in  each  period  of  twenty-five  years.  Yet  even  during  these  periods,  in  some 
of  the  towns,  the  deaths  exceeded  the  births,  a  circumstance  which  clearly  proves 
that  in  those  parts  of  the  country  which  supplied  this  deficiency,  the  increase 
must  have  been  much  more  rapid  than  the  general  average. 

In  the  back  settlements,  where  the  sole  employment  is  agriculture,  and 
vicious  customs  and  unwholesome  occupations  are  little  known,  the  population 
has  been  found  to  double  itself  in  fifteen  years.  Even  this  extraordinary  rate  of 
increase  is  probably  short  of  the  utmost  power  of  population.  Very  severe  labour 
is  requisite  to  clear  a  fresh  country;  such  situations  are  not  in  general  considered 
as  particularly  healthy;  and  the  inhabitants  are  probably  occasionally  subject  to 
the  incursions  of  the  Indians,  which  may  destroy  some  lives,  or  at  any  rate 
diminish  the  fruits  of  their  industry. 

According  to  a  table  of  Euler,  calculated  on  a  mortality  of  1  in  36,  if 
the  births  be  to  the  deaths  in  the  proportion  of  3  to  1,  the  period  of  doubling 
will  be  only  12%  years.  And  this  proportion  is  not  only  a  possible  supposition, 
but  has  actually  occurred  for  short  periods  in  more  countries  than  one. 

Sir  William  Petty  supposes  a  doubling  possible  in  so  short  a  time  as 
ten  years. 

But  to  be  perfectly  sure  that  we  are  far  within  the  truth,  we  will  take 


392  •  APPENDIX 

the  slowest  of  these  rates  of  increase,  a  rate,  in  which  all  concurring  testimonies 
agree,  and  which  has  been  repeatedly  ascertained  to  be  from  procreation  only. 

It  may  safely  be  pronounced,  therefore,  that  population,  when  un- 
checked, goes  on  doubling  itself  every  twenty-five  years,  or  increases  in  a  geo- 
metrical ratio. 

The  rate  according  to  which  the  productions  of  the  earth  may  be  sup- 
posed to  increase,  it  will  not  be  so  easy  to  determine.  Of  this,  however,  we  may 
be  perfectly  certain,  that  the  ratio  of  their  increase  must  be  totally  of  a  different 
nature  from  the  ratio  of  the  increase  of  population.  A  thousand  millions  are  just 
as  easily  doubled  every  twenty-five  years  by  the  power  of  population  as  a  thou- 
sand. But  the  food  to  support  the  increase  from  the  greater  number  will  by  no 
means  be  obtained  with  the  same  facility.  Man  is  necessarily  confined  in  room. 
When  acre  has  been  added  to  acre  till  all  the  fertile  land  is  occupied,  the  yearly 
increase  of  food  must  depend  upon  the  melioration  of  the  land  already  in  pos- 
session. This  is  a  stream,  which  from  the  nature  of  all  soils,  instead  of  increasing, 
must  be  gradually  diminishing.  But  population,  could  it  be  supplied  with  food, 
would  go  on  with  unexhausted  vigour;  and  the  increase  of  one  period  would 
furnish  the  power  of  a  greater  increase  the  next,  and  this  without  any  limit. 

From  the  accounts  we  have  of  China  and  Japan,  it  may  be  fairly 
doubted,  whether  the  best  directed  efforts  of  human  industry  could  double  the 
produce  of  these  countries  even  once  in  any  number  of  years.  There  are  many 
parts  of  the  globe,  indeed,  hitherto  uncultivated,  and  almost  unoccupied;  but  the 
right  of  exterminating,  or  driving  into  a  corner  where  they  must  starve,  even  the 
inhabitants  of  these  thinly  populated  regions,  will  be  questioned  in  a  moral  view. 
The  process  of  improving  their  minds  and  directing  their  industry  would  neces- 
sarily be  slow;  and  during  this  time,  as  population  would  regularly  keep  pace 
with  the  increasing  produce,  it  would  rarely  happen  that  a  great  degree  of  knowl- 
edge and  industry  would  have  to  operate  at  once  upon  rich  unappropriated  soil. 
Even  where  this  might  take  place,  as  it  does  sometimes  in  new  colonies,  a 
geometrical  ratio  increases  with  such  extraordinary  rapidity,  that  the  advantage 
could  not  last  long.  If  America  continue  increasing,  which  she  certainly  will  do, 
though  not  with  the  same  rapidity  as  formerly,  the  Indians  will  be  driven  further 
and  further  back  into  the  country,  till  the  whole  race  is  ultimately  exterminated. 

These  observations  are,  in  a  degree,  applicable  to  all  the  parts  of  the 
earth,  where  the  soil  is  imperfectly  cultivated.  To  exterminate  the  inhabitants  of 
the  greatest  part  of  Asia  and  Africa,  is  a  thought  that  could  not  be  admitted  for 
a  moment.  To  civilize  and  direct  the  industry  of  the  various  tribes  of  Tartars  and 
Negroes,  would  certainly  be  a  work  of  considerable  time,  and  of  variable  and 
uncertain  success. 

Europe  is  by  no  means  so  fully  peopled  as  it  might  be.  In  Europe  there 
is  the  fairest  chance  that  human  industry  may  receive  its  best  direction.  The 
science  of  agriculture  has  been  much  studied  in  England  and  Scotland;  and 


APPENDIX  •  393 

there  is  still  a  great  portion  of  uncultivated  land  in  these  countries.  Let  us  con- 
sider, at  what  rate  the  produce  of  this  island  might  be  supposed  to  increase  under 
circumstances  the  most  favourable  to  improvement. 

If  it  be  allowed,  that  by  the  best  possible  policy,  and  great  encourage- 
ments to  agriculture,  the  average  produce  of  the  island  could  be  doubled  in  the 
first  twenty-five  years,  it  will  be  allowing  probably  a  greater  increase  than  could 
with  reason  be  expected. 

In  the  next  twenty-five  years,  it  is  impossible  to  suppose  that  the  pro- 
duce could  be  quadrupled.  It  would  be  contrary  to  all  our  knowledge  of  the 
properties  of  land.  The  improvement  of  the  barren  parts  would  be  a  work  of 
time  and  labour;  and  it  must  be  evident  to  those  who  have  the  slightest  ac- 
quaintance with  agricultural  subjects,  that  in  proportion  as  cultivation  extended, 
the  additions  that  could  yearly  be  made  to  the  former  average  produce  must  be 
gradually  and  regularly  diminishing.  That  we  may  be  the  better  able  to  compare 
the  increase  of  population  and  food,  let  us  make  a  supposition,  which,  without 
pretending  to  accuracy,  is  clearly  more  favourable  to  the  power  of  production  in 
the  earth,  than  any  experience  we  have  had  of  its  qualities  will  warrant. 

Let  us  suppose  that  the  yearly  additions  which  might  be  made  to  the 
former  average  produce,  instead  of  decreasing,  which  they  certainly  would  do, 
were  to  remain  the  same;  and  that  the  produce  of  this  island  might  be  increased 
every  twenty-five  years,  by  a  quantity  equal  to  what  it  at  present  produces.  The 
most  enthusiastic  speculator  cannot  suppose  a  greater  increase  than  this.  In  a  few 
centuries  it  would  make  every  acre  of  land  in  the  island  like  a  garden. 

If  this  supposition  be  applied  to  the  whole  earth,  and  if  it  be  allowed 
that  the  subsistence  for  man  which  the  earth  affords,  might  be  increased  every 
twenty-five  years  by  a  quantity  equal  to  what  it  at  present  produces,  this  will  be 
supposing  a  rate  of  increase  much  greater  than  we  can  imagine  that  any  possible 
exertions  of  mankind  could  make  it. 

It  may  be  fairly  pronounced  therefore,  that,  considering  the  present 
average  state  of  the  earth,  the  means  of  subsistence,  under  circumstances  the 
most  favourable  to  human  industry,  could  not  possibly  be  made  to  increase  faster 
than  in  an  arithmetical  ratio. 

The  necessary  effects  of  these  two  different  rates  of  increase,  when 
brought  together,  will  be  very  striking.  Let  us  call  the  population  of  this  island 
eleven  millions;  and  suppose  the  present  produce  equal  to  the  easy  support  of 
such  a  number.  In  the  first  twenty-five  years  the  population  would  be  twenty-two 
millions,  and  the  food  being  also  doubled,  the  means  of  subsistence  would  be 
equal  to  this  increase.  In  the  next  twenty-five  years,  the  population  would  be 
forty-four  millions,  and  the  means  of  subsistence  only  equal  to  the  support  of 
thirty-three  millions.  In  the  next  period  the  population  would  be  eighty-eight 
millions,  and  the  means  of  subsistence  just  equal  to  the  support  of  half  of  that 
number.  And  at  the  conclusion  of  the  first  century,  the  population  would  be  a 


394  •  APPENDIX 

hundred  and  seventy-six  millions,  and  the  means  of  subsistence  only  equal  to  the 
support  of  fifty-five  millions,  leaving  a  population  of  a  hundred  and  twenty-one 
millions  totally  unprovided  for. 

Taking  the  whole  earth  instead  of  this  island,  emigration  would  of 
course  be  excluded;  and  supposing  the  present  population  equal  to  a  thousand 
millions,  the  human  species  would  increase  as  the  numbers  1,2,4,8,16,32,64,128, 
256,  and  subsistence  as  1,2,3,4,5,6,7,8,9.  In  two  centuries  the  population  would 
be  to  the  means  of  subsistence  as  256  to  9;  in  three  centuries  as  4096  to  13,  and 
in  two  thousand  years  the  difference  would  be  almost  incalculable. 

In  this  supposition  no  limits  whatever  are  placed  to  the  produce  of  the 
earth.  It  may  increase  for  ever,  and  be  greater  than  any  assignable  quantity;  yet 
still  the  power  of  population  being  in  every  period  so  much  superior,  the  increase 
of  the  human  species  can  only  be  kept  down  to  the  level  of  the  means  of  sub- 
sistence by  the  constant  operation  of  the  strong  law  of  necessity  acting  as  a  check 
upon  the  greater  power. 

Chapter  2  —  of  the  general  checks  to 

POPULATION,     AND     THE      MODE      OF     THEIR 
OPERATION 

The  ultimate  check  to  population  appears  then  to  be  a  want  of  food  arising 
necessarily  from  the  different  ratios  according  to  which  population  and  food 
increase.  But  this  ultimate  check  is  never  the  immediate  check,  except  in  cases  of 
actual  famine. 

The  immediate  check  may  be  stated  to  consist  in  all  those  customs,  and 
all  those  diseases  which  seem  to  be  generated  by  a  scarcity  of  the  means  of  sub- 
sistence; and  all  those  causes,  independent  of  this  scarcity,  whether  of  a  moral  or 
physical  nature,  which  tend  prematurely  to  weaken  and  destroy  the  human  frame. 

These  checks  to  population,  which  are  constantly  operating  with  more 
or  less  force  in  every  society,  and  keep  down  the  number  to  the  level  of  the 
means  of  subsistence,  may  be  classed  under  two  general  heads,  the  preventive, 
and  the  positive  checks. 

The  preventive  check,  as  far  as  it  is  voluntary,  is  peculiar  to  man,  and 
arises  from  that  distinctive  superiority  in  his  reasoning  faculties,  which  enables 
him  to  calculate  distant  consequences.  The  checks  to  the  indefinite  increase  of 
plants  and  irrational  animals  are  all  either  positive,  or,  if  preventive,  involuntary. 
But  man  cannot  look  around  him,  and  see  the  distress  which  frequently  presses 
upon  those  who  have  large  families;  he  cannot  contemplate  his  present  posses- 
sions or  earnings,  which  he  now  nearly  consumes  himself,  and  calculate  the 
amount  of  each  share,  when  with  very  little  addition  they  must  be  divided,  per- 
haps, among  seven  or  eight,  without  feeling  a  doubt,  whether  if  he  follow  the 
bent  of  his  inclinations,  he  may  be  able  to  support  the  offspring  which  he  will 
probably  bring  into  the  world.  In  a  state  of  equality,  if  such  can  exist,  this  would 


APPENDIX  •  395 

be  a  simple  question.  In  the  present  state  of  society  other  considerations  occur. 
Will  he  not  lower  his  rank  in  life,  and  be  obliged  to  give  up  in  great  measure 
his  former  habits?  Does  any  mode  of  employment  present  itself  by  which  he 
may  reasonably  hope  to  maintain  a  family?  Will  he  not  at  any  rate  subject  him- 
self to  greater  difficulties,  and  more  severe  labour  than  in  his  single  state?  Will 
he  not  be  unable  to  transmit  to  his  children  the  same  advantages  of  education  and 
improvement  that  he  had  himself  possessed?  Does  he  even  feel  secure  that, 
should  he  have  a  large  family,  his  utmost  exertions  can  save  them  from  rags  and 
squalid  poverty,  and  their  consequent  degradation  in  the  community?  And  may 
he  not  be  reduced  to  the  grating  necessity  of  forfeiting  his  independence,  and  of 
being  obliged  to  the  sparing  hand  of  charity  for  support  ? 

These  considerations  are  calculated  to  prevent,  and  certainly  do  prevent, 
a  great  number  of  persons  in  all  civilized  nations  from  pursuing  the  dictate  of 
nature  in  an  early  attachment  to  one  woman. 

If  this  restraint  do  not  produce  vice,  it  is  undoubtedly  the  least  evil  that 
can  arise  from  the  principle  of  population.  Considered  as  a  restraint  on  a  strong 
natural  inclination,  it  must  be  allowed  to  produce  a  certain  degree  of  temporary 
unhappiness;  but  evidently  slight,  compared  with  the  evils  which  result  from 
any  of  the  other  checks  to  population;  and  merely  of  the  same  nature  as  many 
other  sacrifices  of  temporary  to  permanent  gratification,  which  it  is  the  business 
of  a  moral  agent  continually  to  make. 

When  this  restraint  produces  vice,  the  evils  which  follow  are  but  too 
conspicuous.  A  promiscuous  intercourse  to  such  a  degree  as  to  prevent  the  birth 
of  children  seems  to  lower  in  the  most  marked  manner  the  dignity  of  human 
nature.  It  cannot  be  without  its  effect  on  men,  and  nothing  can  be  more  obvious 
than  its  tendency  to  degrade  the  female  character,  and  to  destroy  all  its  most 
amiable  and  distinguishing  characteristics.  Add  to  which,  that  among  those  un- 
fortunate females  with  which  all  great  towns  abound,  more  real  distress  and 
aggravated  misery  are  perhaps  to  be  found,  than  in  any  other  department  of 
human  life. 

When  a  general  corruption  of  morals  with  regard  to  the  sex  pervades 
all  the  classes  of  society,  its  effects  must  necessarily  be,  to  poison  the  springs  of 
domestic  happiness,  to  weaken  conjugal  and  parental  affection,  and  to  lessen  the 
united  exertions  and  ardour  of  parents  in  the  care  and  education  of  their  chil- 
dren; effects  which  cannot  take  place  without  a  decided  diminution  of  the  gen- 
eral happiness  and  virtue  of  the  society;  particularly  as  the  necessity  of  art  in  the 
accomplishment  and  conduct  of  intrigues,  and  in  the  concealment  of  their  conse- 
quences, necessarily  leads  to  many  other  vices. 

The  positive  checks  to  population  are  extremely  various,  and  include 
every  cause,  whether  arising  from  vice  or  misery,  which  in  any  degree  contributes 
to  shorten  the  natural  duration  of  human  life.  Under  this  head  therefore  may  be 
enumerated  all  unwholesome  occupations,  severe  labour  and  exposure  to  the 
seasons,  extreme  poverty,  bad  nursing  of  children,  great  towns,  excesses  of  all 


396  •  APPENDIX 

kinds,  the  whole  train  of  common  diseases  and  epidemics,  wars,  plagues,  and 
famine. 

On  examining  these  obstacles  to  the  increase  of  population  which  I  have 
classed  under  the  heads  of  preventive  and  positive  checks,  it  will  appear  that  they 
are  all  resolvable  into  moral  restraint,  vice,  and  misery. 

Of  the  preventive  checks,  the  restraint  from  marriage  which  is  not  fol- 
lowed by  irregular  gratifications  may  properly  be  termed  moral  restraint.  Promis- 
cuous intercourse,  unnatural  passions,  violations  of  the  marriage  bed,  and  im- 
proper arts  to  conceal  the  consequences  of  irregular  connexions,  are  preventive 
checks  that  clearly  come  under  the  head  of  vice. 

Of  the  positive  checks,  those  which  appear  to  arise  unavoidably  from 
the  laws  of  nature  may  be  called  exclusively  misery;  and  those  which  we  obvi- 
ously bring  upon  ourselves,  such  as  wars,  excesses,  and  many  others  which  it 
would  be  in  our  power  to  avoid,  are  of  a  mixed  nature.  They  are  brought  upon 
us  by  vice,  and  their  consequences  are  misery. 

The  sum  of  all  these  preventive  and  positive  checks  taken  together 
forms  the  immediate  check  to  population;  and  it  is  evident  that  in  every  country 
where  the  whole  of  the  procreative  power  cannot  be  called  into  action,  the  pre- 
ventive and  the  positive  checks  must  vary  inversely  as  each  other;  that  is,  in 
countries  either  naturally  unhealthy,  or  subject  to  a  great  mortality,  from  what- 
ever cause  it  may  arise,  the  preventive  check  will  prevail  very  little.  In  those 
countries,  on  the  contrary,  which  are  naturally  healthy,  and  where  the  preventive 
check  is  found  to  prevail  with  considerable  force,  the  positive  check  will  prevail 
very  little,  or  the  mortality  be  very  small. 

In  every  country  some  of  these  checks  are,  with  more  or  less  force,  in 
constant  operation;  yet  notwithstanding  their  general  prevalence,  there  are  few 
states  in  which  there  is  not  a  constant  effort  in  the  population  to  increase  beyond 
the  means  of  subsistence.  This  constant  effort  as  constantly  tends  to  subject  the 
lower  classes  of  society  to  distress,  and  to  prevent  any  great  permanent  meliora- 
tion of  their  condition. 

These  effects,  in  the  present  state  of  society,  seem  to  be  produced  in  the 
following  manner.  We  will  suppose  the  means  of  subsistence  in  any  country  just 
equal  to  the  easy  support  of  its  inhabitants.  The  constant  effort  towards  popula- 
tion, which  is  found  to  act  even  in  the  most  vicious  societies,  increases  the  num- 
ber of  people  before  the  means  of  subsistence  are  increased.  The  food  therefore 
which  before  supported  eleven  millions,  must  now  be  divided  among  eleven  mil- 
lions and  a  half.  The  poor  consequently  must  live  much  worse,  and  many  of 
them  be  reduced  to  severe  distress.  The  number  of  labourers  also  being  above 
the  proportion  of  work  in  the  market,  the  price  of  labour  must  tend  to  fall, 
while  the  price  of  provisions  would  at  the  same  time  tend  to  rise.  The  labourer 
therefore  must  do  more  work,  to  earn  the  same  as  he  did  before.  During  this 
season  of  distress  the  discouragements  to  marriage,  and  the  difficulty  of  rearing 
a  family  are  so  great,  that  population  is  nearly  at  a  stand.  In  the  mean  time,  the 


APPENDIX  •  397 

cheapness  of  labour,  the  plenty  of  labourers,  and  the  necessity  of  an  increased 
industry  among  them,  encourage  cultivators  to  employ  more  labour  upon  their 
land,  to  turn  up  fresh  soil,  and  to  manure  and  improve  more  completely  what  is 
already  in  tillage;  till  ultimately  the  means  of  subsistence  may  become  in  the 
same  proportion  to  the  population,  as  at  the  period  from  which  we  set  out.  The 
situation  of  the  labourer  being  then  again  tolerably  comfortable,  the  restraints  to 
population  are  in  some  degree  loosened;  and,  after  a  short  period,  the  same 
retrograde  and  progressive  movements,  with  respect  to  happiness,  are  repeated. 

This  sort  of  oscillation  will  not  probably  be  obvious  to  common  view; 
and  it  may  be  difficult  even  for  the  most  attentive  observer  to  calculate  its 
periods.  Yet  that  in  the  generality  of  old  states,  some  such  vibration  does  exist, 
though  in  a  much  less  marked,  and  in  a  much  more  irregular  manner,  than  I 
have  described  it,  no  reflecting  man  who  considers  the  subject  deeply  can  well 
doubt. 

One  principal  reason  why  this  oscillation  has  been  less  remarked,  and 
less  decidedly  confirmed  by  experience  than  might  naturally  be  expected,  is,  that 
the  histories  of  mankind  which  we  possess  are,  in  general,  histories  only  of  the 
higher  classes.  We  have  not  many  accounts,  that  can  be  depended  on,  of  the 
manners  and  customs  of  that  part  of  mankind,  where  these  retrograde  and  pro- 
gressive movements  chiefly  take  place.  A  satisfactory  history  of  this  kind,  of  one 
people  and  of  one  period,  would  require  the  constant  and  minute  attention  of 
many  observing  minds  in  local  and  general  remarks  on  the  state  of  the  lower 
class  of  society,  and  the  causes  that  influenced  it;  and  to  draw  accurate  inferences 
upon  this  subject,  a  succession  of  such  historians  for  some  centuries  would  be 
necessary.  This  branch  of  statistical  knowledge  has  of  late  years  been  attended  to 
in  some  countries,  and  we  may  promise  ourselves  a  clearer  insight  into  the  in- 
ternal structure  of  human  society  from  the  progress  of  these  inquiries.  But  the 
science  may  be  said  yet  to  be  in  its  infancy,  and  many  of  the  objects,  on  which 
it  would  be  desirable  to  have  information,  have  either  been  omitted  or  not  stated 
with  sufficient  accuracy.  Among  these  perhaps  may  be  reckoned,  the  proportion 
of  the  number  of  adults  to  the  number  of  marriages;  the  extent  to  which  vicious 
customs  have  prevailed  in  consequence  of  the  restraints  upon  matrimony;  the 
comparative  mortality  among  the  children  of  the  most  distressed  part  of  the 
community,  and  of  those  who  live  rather  more  at  their  ease;  the  variations  in  the 
real  price  of  labour;  the  observable  differences  in  the  state  of  the  lower  classes 
of  society  with  respect  to  ease  and  happiness,  at  different  times  during  a  certain 
period;  and  very  accurate  registers  of  births,  deaths,  and  marriages,  which  are  of 
the  utmost  importance  in  this  subject. 

A  faithful  history,  including  such  particulars,  would  tend  greatly  to 
elucidate  the  manner  in  which  the  constant  check  upon  population  acts;  and 
would  probably  prove  the  existence  of  the  retrograde  and  progressive  movements 
that  have  been  mentioned;  though  the  times  of  their  vibration  must  necessarily 
be  rendered  irregular  from  the  operation  of  many  interrupting  causes;  such  as, 


398  •  APPENDIX 

the  introduction  of  or  failure  of  certain  manufactures,  a  greater  or  less  prevalent 
spirit  of  agricultural  enterprise;  years  of  plenty,  or  years  of  scarcity;  wars,  sickly 
seasons,  poor  laws,  emigration,  and  other  causes  of  a  similar  nature. 

A  circumstance  which  has  perhaps  more  than  any  other  contributed  to 
conceal  this  oscillation  from  common  view  is,  the  difference  between  the  nominal 
and  real  price  of  labour.  It  very  rarely  happens  that  the  nominal  price  of  labour 
universally  falls;  but  we  well  know  that  it  frequently  remains  the  same,  while 
the  nominal  price  of  provisions  has  been  gradually  rising.  This  is,  in  effect,  a 
real  fall  in  the  price  of  labour;  and,  during  this  period,  the  condition  of  the 
lower  classes  of  the  community  must  be  gradually  growing  worse.  But  the  farm- 
ers and  capitalists  are  growing  rich  from  the  real  cheapness  of  labour.  Their 
increasing  capitals  enable  them  to  employ  a  greater  number  of  men;  and,  as  the 
population  had  probably  suffered  some  check  from  the  greater  difficulty  of  sup- 
porting a  family,  the  demand  for  labour,  after  a  certain  period,  would  be  great 
in  proportion  to  the  supply,  and  its  price  would  of  course  rise,  if  left  to  find  its 
natural  level;  and  thus  the  wages  of  labour,  and  consequently  the  condition  of 
the  lower  classes  of  society,  might  have  progressive  and  retrograde  movements, 
though  the  price  of  labour  might  never  nominally  fall. 

In  savage  life,  where  there  is  no  regular  price  of  labour,  it  is  little  to 
be  doubted  that  similar  oscillations  take  place.  When  population  has  increased 
nearly  to  the  utmost  limits  of  the  food,  all  the  preventive  and  the  positive  checks 
will  naturally  operate  with  increased  force.  Vicious  habits  with  respect  to  the  sex 
will  be  more  general,  the  exposing  of  children  more  frequent,  and  both  the 
probability  and  fatality  of  wars  and  epidemics  will  be  considerably  greater;  and 
these  causes  will  probably  continue  their  operation  till  the  population  is  sunk 
below  the  level  of  the  food;  and  then  the  return  to  comparative  plenty  will  again 
produce  an  increase,  and,  after  a  certain  period,  its  further  progress  will  again 
be  checked  by  the  same  causes. 

But  without  attempting  to  establish  these  progressive  and  retrograde 
movements  in  different  countries,  which  would  evidently  require  more  minute 
histories  than  we  possess,  and  which  the  progress  of  civilization  naturally  tends 
to  counteract,  the  following  propositions  are  intended  to  be  proved : 

1.  Population  is  necessarily  limited  by  the  means  of  subsistence. 

2.  Population  invariably  increases,  where  the  means  of  subsistence  in- 
crease, unless  prevented  by  some  very  powerful  and  obvious  checks. 

3.  These  checks,  and  the  checks  which  repress  the  superior  power  of 
population,  and  keep  its  effects  on  a  level  with  the  means  of  subsistence,  are  all 
resolvable  into  moral  restraint,  vice,  and  misery. 

The  first  of  these  propositions  scarcely  needs  illustration.  The  second 
and  third  will  be  sufficiently  established  by  a  review  of  the  immediate  checks  to 
population  in  the  past  and  present  state  of  society.  .  .  . 


Glossary 


Acentric — lacking  a  centromere. 

Adaptation — adjustment  to  environmental  conditions  by  an  organism  or  a  popula- 
tion so  that  it  becomes  more  fit  for  existence  under  the  prevailing  con- 
ditions. 

Adaptive  radiation — the  evolution  from  a  common  ancestry  of  morphologically  and 
ecologically  divergent  types. 

Allele — one  of  a  pair  or  series  of  alternative  forms  of  a  gene,  occupying  the  same 
locus  in  homologous  chromosomes. 

Allesthetic — traits  that  assume  adaptive  significance  via  the  sense  organs  and  nerv- 
ous system  of  other  organisms. 

Allopatric — individuals  or  populations  spatially  isolated  from  one  another. 

Allopolyploid — an  organism  with  more  than  two  sets  of  chromosomes  derived  from 
two  or  more  species  by  hybridization.  At  meiosis,  synapsis  is  primarily  be- 
tween homologous  chromosomes  of  like  origin. 

Ammonites — an  extinct  group  of  mollusks  related  to  the  living  chambered  nautilus. 

Amphiploid — an  allopolyploid. 

Analogous — similar  in  function  but  different  in  structure  and  origin. 

Anaphase — the  stage  in  nuclear  division  during  which  the  daughter  chromosomes 
separate  and  move  from  the  equatorial  plate  to  the  poles  of  the  spindle. 
It  follows  metaphase  and  precedes  telophase. 

Aneuploid — having  a  chromosome  number  that  is  not  an  exact  multiple  of  the 
basic  haploid  number;  heteroploid. 

Angiosperm — the  flowering  plants:  a  class  having  seeds  enclosed  in  an  ovary. 

399 


400  •  GLOSSARY 

Anther — the  pollen-bearing  part  of  the  stamen. 

Anthocyanin — any  of  a  class  of  soluble  glucoside  pigments  of  flowers  and  plants; 
range  in  color  from  red  through  violet  to  blue. 

Apomixis — asexual  reproduction  in  which  the  outward  appearance  of  sexual  repro- 
duction is  retained  but  no  fertilization  occurs. 

Asexual — any  mode  of  reproduction  not  involving  fertilization,  conjugation,  or 
genetic  recombination.  Progeny  have  the  same  genotype  as  the  parent. 

Autopolyploid — an  organism  having  more  than  two  homologous  sets  of  chromo- 
somes in  its  somatic  cells  and  derived  from  a  single  parent  species. 

Autosome — chromosomes  other  than  the  sex  chromosomes,  ordinarily  found  in 
equal  numbers  in  both  males  and  females. 

Back-cross — the  mating  of  a  hybrid  to  one  of  the  parental  types  used  to  produce 

the  hybrid. 
Back  mutation — the  mutation  of  a  mutant  gene  back  to  its  original  state. 
Balanced  lethals — lethal  genes  so  closely  linked  that  crossing  over  is  rare,  the  genes 

remain   in   repulsion,   both   homozygotes  die,   and   only   the  heterozygote 

survives. 
Balanced  polymorphism — two  or  more  distinct  types  of  individuals  coexisting  in 

the  same  breeding  population,  actively  maintained  by  selection. 

Chiasma — a  visible  change  in  pairing  affecting  two  out  of  the  four  chromatids  in  a 
tetrad  or  bivalent  in  the  first  meiotic  prophase.  The  point  of  apparent 
exchange  of  partners  is  the  chiasma. 

Chromatids — half  chromosomes  resulting  from  longitudinal  duplication  of  a  chro- 
mosome, observable  during  prophase  and  metaphase  and  becoming 
daughter  chromosomes  at  anaphase. 

Chromosome — nucleoprotein  bodies  in  the  nucleus,  usually  constant  in  number  for 
any  given  species,  and  bearing  the  genes  in  linear  order. 

Cline — a  geographical  gradient  in  phenotypic  traits. 

Clone — all  the  individuals  descended  from  a  single  individual  by  asexual  repro- 
duction. 

Coelom — the  body  cavity  of  most  higher  Metazoa;  lined  by  a  distinct  epithelium. 

Coincidence — the  ratio  of  observed  double  crossovers  to  expected  double  crossovers 
calculated  on  the  basis  of  independent  occurrence.  This  ratio  is  used  as  a 
measure  of  interference  in  crossing  over. 

Crossing  over — the  exchange  of  corresponding  segments  between  the  chromatids  of 
homologous  chromosomes.  The  result  is  a  recombination  of  genes  between 
two  homologous  groups  of  linked  genes. 

Cytology — the  study  of  the  structure,  physiology,  development,  reproduction,  and 
life  history  of  cells. 

Deficiency — the  absence  or  deletion  of  a  segment  of  a  chromosome. 

Deletion — a  deficiency,  especially  in  which  an  internal  chromosomal  segment  is 
missing. 

Demographic  transition — the  change  from  a  high  birth  rate — high  death  rate  so- 
ciety to  one  with  a  low  birth  rate  and  a  low  death  rate. 

Deuterostomia — animal  groups  in  which  the  blastopore  becomes  the  anus  and  the 
mouth  is  formed  de  novo. 

Differential  reproduction — reproduction  in  which  different  types  do  not  contribute 
to  the  next  generation  in  proportion  to  their  numbers. 


GLOSSARY  •  401 

Diploid — having  two  sets  of  chromosomes.  Somatic  cells  of  higher  plants  and 
animals  derived  from  the  fertilized  egg  are  ordinarily  diploid  in  contrast 
to  the  haploid  gametes. 

DNA — deoxyribonucleic  acid,  the  hereditary  material  in  the  majority  of  species. 

Dominant — an  inherited  trait  expressed  in  the  phenotype,  regardless  of  whether 
the  gene  controlling  it  is  in  the  heterozygous  or  the  homozygous  condition. 
Thus  the  dominant  trait  from  one  parent  is  expressed  in  a  hybrid  but  the 
recessive  trait,  though  transmitted,  is  not  expressed.  Also  a  group  of  ani- 
mals or  plants  that  is  pre-eminent  in  a  given  region  or  at  a  given  time. 

Doubling  dose — the  dose,  usually  of  radiation,  sufficient  to  cause  a  number  of  muta- 
tions equal  to  that  occurring  spontaneously. 

Duplication — the  occurrence  of  a  chromosome  segment  more  than  once  in  the  same 
chromosome  or  haploid  genome. 

Dysgenic — tending  to  be  harmful  to  the  hereditary  qualities  of  a  species. 

Ecological  niche — the  place  occupied  by  a  species  in  the  community  structure  of 
which  it  is  a  part. 

Ecotype — an  ecological  race  whose  genotype  is  adapted  to  a  particular  restricted 
habitat  as  the  result  of  natural  selection.  Many  plant  species  have  distinct 
ecotypes  on  the  sea  coast,  in  the  desert,  or  in  the  mountains. 

Effective  size  of  population — the  number  of  individuals  in  a  local  breeding  popu- 
lation that  actually  contribute  genes  to  the  next  generation. 

Embryo  sac — the  mature  female  gametophyte  in  higher  plants. 

Endosperm — the  nutritive  tissue,  typically  triploid,  arising  from  double  fertiliza- 
tion by  the  second  male  nucleus  of  two  of  the  eight  nuclei  of  the  embryo  sac. 

Enzyme — protein  catalyst  in  living  organisms,  typically  formed  from  a  protein  part 
(apoenzyme)  conferring  specificity  and  a  nonprotein  part  (coenzyme) 
necessary  for  activity. 

Epigamic — promoting  the  union  of  gametes. 

Epistasis — the  suppression  of  the  expression  of  a  gene  or  genes  by  other  genes  not 
allelic  to  the  genes  suppressed.  Similar  to  dominance  but  involving  the 
interaction  of  nonallelic  genes.  Sometimes  used  to  refer  to  all  nonallelic 
interactions. 

Ethology — the  study  of  animal  behavior. 

Euploid — an  exact  multiple  of  the  haploid  chromosome  number. 

Eutheria — the  placental  mammals. 

Fertilization — the  fusion  of  gametes  to  form  a  zygote. 

Finalism — the  concept  that  the  world  is  directed  toward  a  definite  purposive  goal. 

Fitness — the  number  of  offspring  left  by  an  individual  as  compared  with  the  average 

of  the  population  of  which  it  is  a  member  or  compared  to  individuals  of 

different  genotypes. 
Flame  bulb — a  cup-shaped  mass  of  protoplasm  bearing  a  tuft  of  cilia  projecting  into 

the  cavity  of  the  cup,  found  at  the  closed  inner  end  of  a  protonephridium. 
Founder  principle — the  concept  that,  when  a  small  population  invades  a  new  area, 

evolutionary  divergence  may  be  hastened  not  only  because  of  the  new  and 

probably  different  selection  pressures  but  also  because,  due  to  sampling, 

the  gene  pool  of  this  small  group  may  differ  in  significant  ways  from  that 

of  the  parental  population. 

Gamete — a  sex  cell. 

Gametogenesis — the  formation  of  gametes. 


402  •  GLOSSARY 

Gametophyte — the  gamete-forming  haploid  generation  in  higher  plants. 

Gene — a  Mendelian  factor  or  unit  of  inheritance  that  occupies  a  fixed  chromosomal 
locus,  is  transmitted  in  the  germ  cells,  and,  interacting  with  other  genes, 
the  cytoplasm,  and  the  environment,  controls  the  development  of  a  char- 
acter. 

Gene  flow — the  spread  of  genes  from  one  breeding  population  to  others  as  the  result 
of  migration. 

Gene  frequency — the  proportion  between  one  particular  type  of  allele  and  the  total 
of  all  alleles  at  this  locus  in  a  breeding  population. 

Gene  pool — the  sum  total  of  the  genes  in  a  given  breeding  population. 

Genetic  drift — changes  in  gene  frequency  in  small  breeding  populations  due  to 
random  fluctuations. 

Genetic  isolate — a  breeding  population  not  exchanging  genes  with  any  other  group. 

Genetic  system — the  way  in  which  the  genetic  material  is  organized  and  transmitted 
from  one  generation  to  the  next. 

Genome — the  chromosome  complement  of  a  gamete;  also,  of  a  zygote. 

Genotype — the  entire  genetic  constitution  of  an  organism. 

Gynandromorph — an  individual  with  both  male  and  female  sectors;  a  sexual 
chimaera. 

Haploid — having  only  a  single  set  of  chromosomes. 

Hardy- Weinberg  law — in  a  large  random  mating  population  in  the  absence  of  mu- 
tation and  selection,  gene  frequencies  remain  constant. 

Hermaphrodite — an  individual  with  functional  ovaries  and  testes. 

Heterogametic — producing  unlike  gametes,  especially  with  regard  to  the  sex  chro- 
mosomes. Where  the  male  is  XY,  he  is  heterogametic. 

Heteromorphic — having  more  than  one  form. 

Heteroploid — having  a  chromosome  number  that  is  not  an  exact  multiple  of  the 
basic  haploid  number;  aneuploid. 

Heterosis — hybrid  vigor. 

Heterozygous — having  different  alleles  at  one  or  more  loci. 

Hexaploid — having  six  haploid  sets  of  chromosomes. 

Homeostasis — a  dynamic  equilibrium  in  a  biological  system. 

Homologous — 1.  similarity  of  structure  due  to  similar  hereditary  and  developmental 
origin;  2.  chromosomes  in  which  the  same  gene  loci  occur  in  the  same 
sequence. 

Homozygous — having  any  specified  gene  or  genes  present  in  double  dose  so  that 
the  organism  breeds  true  at  these  particular  gene  loci. 

Inbred — the  result  of  matings  between  relatives. 

Incompatibility — the  inability  of  pollen  to  fertilize  due  to  failure  of  the  pollen  tube 
to  grow  normally  in  the  style. 

Independent  assortment — segregation  of  one  factor  pair  occurring  independently  of 
the  segregation  of  other  factor  pairs. 

Industrial  melanism — the  appearance  of  dark  or  melanistic  forms  of  a  species  in 
industrial  regions. 

Interference — the  effect  by  which  the  occurrence  of  one  cross-over  reduces  the  prob- 
ability of  another  occurring  in  its  vicinity. 

Interphase — the  "resting"  stage,  used  especially  in  referring  to  the  phase  between 
the  two  meiotic  divisions. 

Intersex — an  individual  with  traits  intermediate  between  those  of  males  and  females. 


GLOSSARY  •  403 

Introgressive  hybridization — the  addition  of  genes  from  one  species  to  the  gene 
pool  of  another  species  through  hybridization  and  back-crossing. 

Inversion — rotation  of  a  chromosome  segment  through  180  degrees  so  that  the 
linear  order  of  the  genes  is  reversed  relative  to  the  rest  of  the  chromosome. 

Isoalleles — alleles  so  similar  in  their  effects  that  special  techniques  are  needed  to 
distinguish  between  them. 

Isolating  mechanism — any  intrinsic  factor  that  prevents  or  reduces  interbreeding  be- 
tween two  populations. 

Isomorphic — having  similar  form. 

Lamarckism — usually,  the  theory  of  the  inheritance  of  acquired  characteristics. 

Lethal — a  gene  or  genotype  that,  when  expressed,  is  fatal  to  its  bearer. 

Linkage — the  association  of  genes  in  inheritance  due  to  their  being  on  the  same 

chromosome.    Genes  borne  on  homologous  chromosomes  belong  to  the 

same  linkage  group. 
Locus  (pi.,  loci) — the  position  of  a  gene  on  a  chromosome. 

Materialism — any  theory  that  considers  the  nature  of  the  universe  to  be  sufficiently 

explained  by  the  existence  and  nature  of  matter. 
Mean— the  sum  of  a  group  of  observations  divided  by  the  number  in  the  group. 
Mechanist — one  who  regards  the  phenomena  of  nature  as  the  effects  of  merely 

mechanical  forces. 
Megaspore — the  larger  of  the  two  kinds  of  haploid  spores  produced  by  hetero- 

sporous  plants.  In  seed  plants  the  megaspore  gives  rise  to  the  embryo  sac, 

the  female  gametophyte. 
Meiosis — the  reduction  divisions  during  which  the  chromosome  number  is  reduced 

from  diploid  to  haploid;  two  nuclear  divisions  during  which  the  chromo- 
somes divide  only  once. 
Mendel's  laws — segregation  and  independent  assortment. 
Metabolism — the  sum  total  of  the  chemical  processes  in  living  cells  by  which  energy 

is  provided,  new  materials  assimilated  or  synthesized,  and  wastes  removed. 
Metamorphosis — a  more  or  less  abrupt  change  in  the  form  of  an  animal  after  the 

embryonic  period. 
Metanephridia — nephridia  (excretory  organs)  with  open  inner  ends. 
Metaphase — the  stage  of  nuclear  division  during  which  the  chromosomes  lie  in  the 

equatorial  plane  of  the  spindle;   after  prophase  and  prior  to  anaphase. 
Microspore — the  smaller  of  the  two  kinds  of  haploid  spores  produced  by  hetero- 

sporous  plants.  In  seed  plants  the  microspore  gives  rise  to  the  pollen  grain, 

the  male  gametophyte. 
Mitosis — the  process  by  which  the  nucleus  is  divided  into  two  daughter  nuclei,  each 

with  a  chromosome  complement  similar  to  that  of  the  original  nucleus. 
Modifying  factor — a  gene  that  affects  the  expression  of  another  nonallelic  gene. 

Often  without  other  known  effects. 
Monohybrid — a  cross  involving  parents  that  differ  with  respect  to  a  single  specific 

gene. 
Monosomic — a  diploid  with  one  chromosome  missing  from  the  chromosome  com- 
plement. 
Multiple  alleles — a  series  of  more  than  two  alternative  forms  of  a  gene  at  a  single 

locus. 
Multiple  factors — two  or  more  pairs  of  factors  with  a  similar  or  complementary 

cumulative  effect  on  a  single  trait. 


404  •  GLOSSARY 

Mutagenic — capable  of  inducing  mutations. 

Mutation — in  the  broad  sense,  any  sudden  change  in  the  hereditary  material,  includ- 
ing both  "point"  or  gene  mutations  and  chromosomal  rearrangements.  In 
the  narrow  sense,  point  mutations  only. 

Mutation  pressure — the  continued  recurrent  production  of  a  gene  by  mutation,  tend- 
ing to  increase  its  frequency. 

Mutation  rate — the  frequency  with  which  a  particular  mutation  occurs.  Also  the 
frequency  of  all  mutations  in  a  given  population. 

Mutation  rate  gene — a  gene  that  influences  the  mutation  rate  of  genes  at  other  loci. 

Nephridium — an  excretory  tubule. 

Normal  curve — a  symmetrical  bell-shaped  curve  often  approximated  when  fre- 
quency distributions  are  plotted  from  observations  on  biological  materials. 

Octoploid — a  polyploid  with  eight  haploid  sets  of  chromosomes. 

Oocyte — primary :  egg  mother  cell  giving  rise  by  the  first  meiotic  division  to  the 
secondary  oocyte  and  the  first  polar  body.  The  secondary  oocyte  at  the 
second  meiotic  division  gives  rise  to  the  ovum  and  to  a  second  polar  body. 

Oogonium — a  cell  giving  rise  to  primary  oocytes  by  mitosis. 

Orthogenesis — evolution  more  or  less  continuously  in  a  single  direction  over  a  long 
span  of  time.  Often  used  with  vitalistic  implications.  , 

Orthoselection — natural  selection  acting  continuously  in  the  same  direction  over 
long  periods  of  time.  Often  used  in  place  of  orthogenesis  to  avoid  impli- 
cation of  vitalism. 

Overdominance — the  superiority  of  the  heterozygote  over  both  types  of  homo- 
zygotes. 

Paracentric — an   inversion   that  does   not  include   the  centromere,   but   is  entirely 

within  one  arm  of  the  chromosome. 
Parthenogenesis — the  development  of  a  new  individual  from  a  germ  cell  (usually 

female)   without  fertilization.  May  be  either  haploid  or  diploid. 
Pericentric — an    inversion   that   includes   the  centromere;    hence  both  chromosome 

arms  are  involved. 
Phenocopy — environmentally   induced  nonhereditary  phenotypic  imitations  of  the 

effects  of  mutant  genes. 
Phenotype — the  sum   total  of  the  observable  or  measurable  characteristics  of  an 

organism  without  reference  to  its  genetic  nature. 
Photosynthesis — the    synthetic   metabolism   carried   on    by    the   chlorophyll-bearing 

tissues  in  plants. 
Phyletic  evolution — evolution  by  a  related  group  of  species  within  a  broad  adaptive 

zone,  carried  on  at  moderate  rates  and  without  marked  change  of  adap- 
tive type. 
Phylogeny — the  evolutionary  history  of  a  taxonomic  group. 

Pistil — in  flowers,  the  female  portion — the  ovary,  style,  and  stigma,  collectively. 
Pleiotropic — a  single  gene  influencing  more  than  one  character. 
Polar  body — in  oogenesis,  the  smaller  cells  produced  during  meiosis  that  do  not 

develop  into  functional  egg  cells. 
Polygene — originally  associated  with  a  particular  theory  of  quantitative  inheritance 

but  now  frequently  used  as  a  synonym  for  multiple  factor. 
Polymorphic — two  or  more  recognizably   different   sorts  of   individuals  within   a 

single  breeding  population. 


GLOSSARY  •  405 

Polyploid — an  organism  with  more  than  two  haploid  sets  of  chromosomes. 
Polysaccharide — a  molecule  formed  by  the  condensation  of  a  number  of  simple 

sugar  molecules — for  example,  starch,  cellulose. 
Polytypic — generally,  a  species  composed  of  several  geographic  races  or  subspecies. 
Position  effect — change  in  the  effect  of  a  gene  due  to  a  change  in  its  position  with 

respect    to    other   genes    in    the   genotype    as   the   result   of   chromosomal 

rearrangement. 
Preadaptation — a  characteristic  that  enables  an  organism  to  be  adapted  to  environ- 
mental conditions  to  which  it  has  not  yet  been  exposed. 
Preformation — the  concept  that  the  individual  is  present  in  miniature  in  the  embryo 

and  that  development  to  adulthood  involves  growth  but  not  differentiation. 
Prophase — the  first  stage  of  nuclear  division. 
Protonephridia — nephridia  with  closed  inner  ends. 

Protostomia — those  animal  groups  in  which  the  blastopore  becomes  the  mouth. 
Pseudoalleles — very    closely    linked   genes,    usually    affecting    the    same    trait,    and 

showing  a  mutant  phenotype  rather  than  the  wild  type  when  in  repulsion 

in  heterozygotes. 
Pseudocoelom — a  body  cavity  not  lined  with  epithelial  cells. 

Quantum  evolution — relatively  rapid  evolution  involving  a  major  adaptive  shift. 

Race — a  subspecies  or  a  geographical  subdivision  of  a  species.  A  geographically 
defined  group  of  breeding  populations  that  differs  from  other  similar 
groups  in  the  frequency  of  one  or  more  genetically  determined  traits. 

Random  mating — the  situation  when  any  individual  of  one  sex  has  an  equal  prob- 
ability of  mating  with  any  individual  of  the  opposite  sex. 

Recapitulation — the  theory  that  ontogeny  recapitulates  phylogeny;  that  is,  that  the 
development  of  the  individual  passes  through  phases  resembling  the  adult 
forms  of  its  successive  ancestors. 

Recessive — an  inherited  trait  only  expressed  in  the  phenotype  when  the  allele  con- 
trolling it  is  in  the  homozygous  condition.  Thus  a  recessive  trait  is  not 
expressed  in  a  hybrid. 

Reciprocal  cross — a  second  cross  similar  to  the  first  but  with  the  sexes  of  the  parents 
interchanged. 

Repeat — a  duplication. 

Reproductive  isolation — inherent  blocks  to  crosses  between  members  of  different 
breeding  populations. 

Roentgen  (r) — the  unit  of  measurement  of  dosage  for  ionizing  radiation.  Equal  to 
the  amount  of  radiation  that  in  air  at  STP  will  produce  2.1  X  109  ion 
pairs  per  cubic  centimeter  or  in  tissue  approximately  two  ionizations  per 
cubic  micron. 

Saprophyte — any  organism  living  on  dead  or  decaying  organic  material. 

Segmental   allopolyploid — an   allopolyploid  in  which  some  chromosome  segments 

from  the  parent  species  are  still  homologous. 
Segregation — the   separation   of   maternal   from  paternal  chromosomes   at  meiosis 

and  hence  the  basis  for  Mendel's  first  law. 
Semilethal — a  gene  or  genotype  that,  when  expressed,  reduces  the  viability  of  its 

bearers  to  less  than  half  of  that  of  the  "normal"  or  standard  type. 
Serology — the  study  through  antigen-antibody  reactions  of  the  nature  and  specificity 

of  antigenic  materials  from  different  sources. 


406  •  GLOSSARY 

Sex  chromosomes — chromosomes  that  are  particularly  involved  in  sex  determination. 
Sex  reversal — a  change  in  the  sexual  character  of  an  individual  from  male  to  female 

or  vice  versa. 
Sexual — a   mode  of   reproduction   normally   involving  of   fusion   of  gametes   and 

genetic  recombination. 
Sexual    isolation — reproductive    isolation    due    to    a    tendency    toward    homogamic 

mating. 
Sexual  selection — selection  based  on  male  competition  or  female  choice  and  respon- 
sible for  sexual  dimorphism. 
Solenocyte — a  long  tubular  cell  with  a  flagellum  at  the  base  of  the  tube  that  extends 

into  the  tube  and  forms  the  closed  end  of  a  protonephridial  tubule. 
Somatic — referring  to  the  body  tissues,  as  contrasted  with  the  germinal  tissues  that 

give  rise  to  the  germ  cells. 
Speciation — the  process  by  which  new  species  are  formed.  In  the  restricted  sense, 

the  splitting  of  one  species  into  a  number  of  different  contemporaneous 

species. 
Spermatid — the  haploid  cell  that  results  from  meiosis  and  develops  into  a  functional 

spermatozoan  without  further  nuclear  division. 
Spermatocyte — primary:   a  sperm  mother  cell  giving  rise  by  the  first  meiotic  divi- 
sion to  two  secondary  spermatocytes.  The  secondary  spermatocytes  at  the 

second  meiotic  division  give  rise  to  four  haploid  spermatids. 
Spermatogonium — a  cell  giving  rise  to  primary  spermatocytes  by  mitosis. 
Spontaneous  generation — the  direct  formation  of  living  organisms  from  nonliving 

matter. 
Sporophyte — the  spore-forming  diploid  generation  in  higher  plants. 
Stamen — in  flowers,  the  male  portion — the  anther  containing  the  pollen  plus  the 

filament  or  stalk. 
Standard  deviation — the  square  root  of  the  sum  of  the  deviations  from  the  mean 

squared   and   divided   by  one   less   than   the   number  of  observations.   A 

measure  of  the  variability  of  a  population  of  individuals. 
Standard  error — the  standard  deviation  divided  by  the  square  root  of  the  number 

of  observations.  A  measure  of  the  variation  of  a  population  of  means. 
Subspecies — see  Race. 
Subvital — a  gene  or  genotype  that,   when  expressed,   reduces  the  viability  of  its 

bearers  significantly  below  that  of  the  "normal"  or  standard  type  but  has 

a  viability  at  least  half  as  great. 
Supervital — a  gene  or  genotype  that,  when  expressed,  is  significantly  more  viable 

than  the  "normal"  or  standard  type. 
Sympatric — coexisting  in  the  same  area,  with  the  implication  that  crossing  is  at  least 

possible. 
Synapsis — the  pairing  of  homologous  chromosomes  of  maternal  and  paternal  origin 

during  the  first  meiotic  prophase.  Also  observed  occasionally  in  somatic 

cells — for  example,  salivary  gland  chromosomes  in  Drosophila. 
Systematics — taxonomy.  The  classification  of  organisms. 
Systemic  mutation — mutations  of  major  effect  presumed  to  give  rise  to  new  species 

or  higher  categories  at  a  single  step. 

Teleology — the  concept  that  evolution  is  purposeful  and  is  directed  toward  some 

definite  goal. 
Telophase — the  last  phase  of  nuclear  division,  following  anaphase,  during  which 

the  daughter  nuclei  are  formed  and  separate  cells  are  formed. 


GLOSSARY  •  407 

Test  cross — a  cross  between  a  presumed  heterozygote  and  a  recessive  homozygote. 
Tetraploid — a  polyploid  with  four  haploid  sets  of  chromosomes. 
Transduction — genetic  recombination  in  bacteria  mediated  by  bacteriophage. 
Transformation— genetic  recombination  in  bacteria  brought  about  by  the  addition 

of  DNA  from  a  different  strain  to  the  culture. 
Transient  polymorphism — temporary  polymorphism  observed  while  one  adaptive 

type  is  replacing  another. 
Translocation — change  in  position  of  a  chromosome  segment  to  another  part  of  the 

same  chromosome  or  to  a  different  chromosome.  Reciprocal — the  exchange 

of  segments  between  two  chromosomes. 
Triploid — a  polyploid  with  three  haploid  sets  of  chromosomes. 
Trisomic — an  organism,  otherwise  diploid,  that  has  three  chromosomes  of  one  type. 

Variance — the  mean  squared  deviation  from  the  mean.  The  square  of  the  standard 

deviation. 
Vitalism — the  concept  that  living  organisms  are  animated  by  a  vital  principle  or 

force  distinct  from  physical  forces. 

Wild  type — the  customary  phenotype.  Also  the  most  frequent  allele  in  wild  popu- 
lations. 

Zygote — the  cell  produced  at  fertilization  by  the  union  of  gametes.  Also  the  indi- 
vidual derived  from  this  cell. 


Index 


Index 


ABO  blood  groups,  178,  265,  342f. 

Acanthocephala,  132f. 

Acoela,  131 

Actinopterygii,  44 

Adalia  bipunctata,  254 

adaptation,   3ff.,    15,    113ff.,   239f.,   301, 

303f.;  individual,  4,  245;  population, 

5,  245 
adaptive  behavior,  8,  10,  12 
adaptive  neutrality,  250 
adaptive  radiation,  42 
adenosine  triphosphate  (ATP),  64f.,  104 
Agassiz,  L.,  32,  42 
Agelaius  phoeniceus,  269 
age  of  earth,  41,  5 If. 
Age  of  Fishes,  42 
Age  of  Mammals,  42 
Age  of  Reptiles,  42 
age  of  universe,  5 iff. 


Agnatha,  44 
agriculture,  348 
albinism,  172f. 
algae,  l44ff. 
allantois,  46 
allesthetic  traits,  3l6f. 
allopatric,  270 

allopolyploidy,   157,  205,  313 
alternation  of  generations,  188f. 
Ambystoma,  92 
American  Indians,  265 
amino  acid  synthesis,  62 
amnion,  46,  92 
amniotes,  92,  120 
amoebae,  125 
amphibians,  44f.,  92,  110 
Amphineura,   135 
Amph'ioxus,  142 
amphiploidy,   157,  205 
analogy,  95ff.,  103 


411 


412  •  INDEX 


anaphase,  186 
Anaxagoras,  15 
Anaximander,  I4f.,  57 
Ancon  sheep,  207 
aneuploidy,  204 
Angiospermae,  I44f.,  152 
anisogametes,  306 
Annelida,  134,  136 
anthropoid  apes,  327 
Anthropoidea,  325ff. 
antibiotics,  242 

antigen-antibody  reactions,  HOf. 
aortic  arches,  88 
apomixis,  314 
aposematic  coloration,  10 
Aquinas,  St.  Thomas,  l6f. 
Arachnida,  111,  139 
archetype,  95 

Aristotle,  15f.,  18,  23,  57,  80,  83 
Arrhenius,  59 
Arthropoda,  132,  136,  139 
artificial  selection,  158,  24 If. 
Aschelminthes,   133 
asexuality,  303f.,  3l4f. 
astaxanthin,  107,  109 
atomic  theory,   16 
Auerbach,  35 
Augustine,  St.,  16 
Australian,  69 
Australoid,  343,  345 
Australopithecus,  335,  338 
autocatalysis,  64,  66 
autopolyploidy,  204 
autosome,  191 
autotrophic,  66,  148 
Aves,  46 

A  vitamins,  107ff. 
axolotl,  92 

B 

back  cross,   173 

Bacon,  Sir  Francis,  18 

bacteria,  l44f.,  I48f. 

bacteriophage,  244,  302 

balanced  lethals,  227,  256,  313 

balanced  polymorphism,  250,  254ff.,  342 


balance  theory  of  sex  determination,  308 

Baldwin  effect,  244f. 

Bar  eye,  203 

barnacle,  58 

barriers,  269 

Bateson,  W.,  34,  158,  195,  211,  253 

Beagle,  26f. 

Bennettitales,  152 

binocular  vision,  325 

binomial,  236 

binomial  system,  19,  81ff. 

biogenetic  law,  87 

biogeographical  realms,  69ff. 

biological  success,  10 

bipedal  locomotion,  330,  335 

bipinnaria  larva,  140 

birds,  46 

Biscutella  laevigata,  282 

bisexual  species,  306 

Bis  ton  betularia,  251 

blastaea,  130 

blastopore,  132 

blastula,  88,  130 

blue  babies,  88 

blue-green  algae,  l44ff. 

Blyth,  E.,  23 

Bohr  effect,  121 

Bonellia,  306f. 

Botallus,  duct  of,  89 

Boyden,  A.  A.,  Ill 

brachiation,  327,  329 

Brachiopoda,  135,  140 

brachyury,   180 

Bridges,  C.  B.,  308 

Bronze  Age,  345,  348 

Broom,  R.,  335 

brown  algae,   145,  148 

Bryophyta,  I44f.,  148,  150f. 

Bryozoa,  134 

Buffon,  G.  L.  L.  de,  17,  19,  21,  23 

Bufo,  287 

Bushmen,  343,  345 


Carnivora,  80 
carotenoids,   106f.,  146 


INDEX  •  413 


cataclysmic  evolution,  284 

catarrhine,  327,  329 

Caucasoid,  343 

cellular  fusion,  302 

Cenozoic,  4 If. 

centromere,  190 

Cercopithecidae,  324,  327,  335 

cerebrum,  327 

cervical  vertebrae,  97f. 

Cesalpino,  17 

Chaetognatha,  139 

chain  of  being,  15,  18f.,  23 

Chambers,  R.,  23 

chemical  evolution,  59ff. 

chemical  mutagens,  209 

chiasmata,  190,  314 

chimpanzee,  329 

chlorophyll,   105 

Chlorophyta,  145,  148,  150f. 

Choanichthyes,  44 

choanoflagellates,  128 

Chondrichthyes,  44 

Chordata,  90,  132,  136,  139,  I4lf. 

chromatid,   186,   190 

chromatophores,  7 

chromosome,  186,  188,  190,  303 

chromosome  homology,  159 

chromosome  map,  197 

chromosome  rearrangements,  199ff. 

chrysomonads,   125 

Chrysophyta,  l45f.,  150 

Ciliata,  125 

cinquefoil,  228,  230,  272 

cis-trans,  204 

Clausen,  Keck,  and  Hiesey,  272 

classification  of  plants,  l44f. 

climate  and  evolution  theory,  75 

cline,  271 

clover,  182f. 

club  mosses,  I44f.,  152 

coadaptation,  258 

Coelenterata,  128ff. 

coelom,  134ff. 

coincidence,  198 

colchicine,  158,  204 

comb  jellies,  129 


comparative  anatomy,  19,  22,  95fF. 

competition,  6,  240 

conifers,  145,  152 

conjugation,  306 

Continental  Drift,  75 

continental  islands,  75f. 

continuous  variation,  2l6ff. 

convergent  evolution,  97 

cooperation,  6,  241 

corn,  211,  218,  220 

Correns,  C,  33 

cosmology,  5  Iff. 

cosmozoa,  59 

countershading,  8,  10 

coupled  reaction,  64 

Cro-Magnon  man,  338ff. 

crossing  over,  195ff. 

Crossopterygii,  44 

crossveinless  condition,  244f. 

Crustacea,  93,  139 

cryptic  coloration,  7,  10,  12,  318 

cryptomonads,   I45f.,  150 

Ctenophora,  129,  131 

cultural  evolution,  345ff. 

Cuvier,  G.,  22f.,  42,  95 

cyanide,  182f. 

Cyanophyta,   I45f. 

Cycadofilicales,  152 

Cynips,  270 

D 

Dart,  R.,  335 

Darwin,  Charles,  20,  23,  25fT.,  69,   83, 

95,  158,  164,  166,  239 
Darwin,  Erasmus,  21,  23 
Darwin,  Robert,  25f. 
Datura,  229 
da  Vinci,  Leonardo,  17 
DDT,  242 
deficiency,  199 
deletion,  199 
De  Maillet,  20 
de  Maupertius,  19 
Democritus,  16 

demographic  transition,  364,  367f. 
demography,  363ff. 


414  •  INDEX 


deoxyribonucleic  acid  (DNA),  66,  150, 

I60f.,  302 
Descartes,  R.,  18,  57 
Deuterostomia,  132,  135,  139f. 
de  Vries,  H.,  33,  213f. 
developmental  homeostasis,  258 
diabetes,  260 
diatoms,   I45f. 

differential  reproduction,  239 
dihybrid,  175 
dinoflagellates,  I45f.,  150 
dinosaurs,  46 
dipleurula  larva,  140 
diploid,   188 
diploidy,  304f. 
Dipnoi,  44,   118 
Diptera,  101,  295 
discontinuous  traits,  216 
disruptive  coloration,  7 
distribution  of  species,  268f. 
Dobzhansky,  Th.,  289,  292 
domestication,   158f. 
dominance,  169,  252ff. 
dominance  theory  of  heterosis,  221 
Doppler  effect,  53 
double  fertilization,  190 
doubling  dose,  357 
Drosophila,  100f.,  196,  202,  226,  228f., 

249,  255,  257ff.,  288,  293,  308ff.,  312, 

353 
Dryopithecus,  332,  335 
Dubinin,  259 
Dubois,  336 
duplication,  199,  201 


East,  E.  M.,  218,  223 
Echinodermata,  111,   139ff. 
Echiurida,  134,  137 
ecological  isolation,  286 
ecological  niche,  10,  42,  68 
ecotype,  228 
Ectoprocta,  134f.,  140 
effective  population  size,  264f. 
elasmobranchs,  117,  120 
elements,  60 


embryo  culture,  288 
embryo  sac,  190 
Embryophyta,  150f. 
emigration,  367 
Empedocles,  15 
Encyclopedists,  17 
endosperm,   190 
Entoprocta,  134 
environment,  5ff. 
Epicurus,  16 
epigamic,  317 
epistasis,  182 
Equidae,  46ff.,  297 
Escherichia  coli,  302 
Ethiopian,  69,  71 
ethology,  287,  318 
eugenics,  376ff. 
Euglena,  107,  124 
Euglenophyta,  145,  148,  150 
Eumycophyta,  145,  149 
Eutheria,  46,  323 
evolving  universe,  53ff. 
excretion,  115ff. 
eye,  96f.,  325 


Felidae,  80 

Felis,  80,  100 

female  choice,  3l6f. 

fermentation,  65 

ferns,  145,  152 

finalism,  43 

Fisher,  R.  A.,  35,  163,  223,  253 

Flagellata,  124ff.,  150 

flatworms,  129ff. 

Flemming,  33 

flowering  plants,  145,  152 

fossil  record,  330fT. 

founder  principle,  271 

freemartin,  311 

fungi,  l44ff. 


galaxies,  53,  55 
Galen,  16 


INDEX  •  415 


Gale  op  sis,  231,  282 

Galton,  34 

gametogenesis,  188 

gametophyte,  189 

Gamow,  G.,  55 

gastraea  theory,  130 

Gastrotricha,  134 

gastrula,  88,  130 

Gegenbauer,  K.,  32 

gene,  171 

gene  flow,  237,  279 

gene  frequency,  235ff. 

gene  homology,  100f.,  159f. 

gene  pool,  270,  298 

generalized  forms,  43f. 

genetic  drift,  237,  263ff. 

genetic  homeostasis,  258 

genetic  recombination,  301n\,  313ff. 

genetic  systems,  301  ff. 

genotype,  172;  frequency  of,  235f. 

geological  column,  40f. 

Gephyrea,  134 

germ  line  theory,  33 

gibbon,  327 

Giles,  212 

gill  arches,  90 

glaciation,  73 

golden  brown  algae,  l45f. 

Goldschmidt,  R.  B.  G.,  101 

Gondwana,  75 

goose  tree  legend,  58 

gorilla,  327,  329 

grackles,  84 

graptolites,  142 

Gray,  A.,  30,  32 

green  algae,  145,  148,  151 

guinea  pig,  100 

Gymnospermae,  145,  152 

gynandromorph,  310 


Hardy-Weinberg,  33,   164,  235ff.,  250 

Harvey,  W.,  17f.,  57 

Hemichordata,   111,  14 If. 

Hemizonia  angustifolia,  272 

Henslow,  J.  S.,  26 

heredity  vs.  environment,  168 

hermaphroditism,  306,  313f. 

heterogametic,  307f. 

heteromorphic,  305 

heteroploidy,  204 

heterosis,  155,  220ff.,  256ff. 

heterotrophic,  66,  I48f. 

heterozygous,  171 

heterozygous  sporophyte,   308 

Holarctic,  69,  73,  75 

homeostasis,  7 

homeotic  mutants,  101 

Hominidae,  327ff. 

Hominoidea  327ff. 

Homo,  336ff. 

homology,  95ff.,  103 

homozygous,  171 

honey  bee,  310 

Hooker,  Sir  Joseph,  29,  32 

horn  worts,  145 

horseshoe  crab,   111 

horsetails,  145,   152 

Hutton,  J.,  20,  23 

Huxley,  T.  H.,  32 

hybrid,   170 

hybrid  breakdown,  288 

hybrid  inviability,  288 

hybrid  sterility,  155,  288 

hybridization,  155ff.,  277ff. 

Hylobates,  332 

Hylocichla,  80 

Hylodes,  91 

hyoid,  90 

hyomandibular,  90 


H 

Haeckel,  E.,  32,  87f.,  93,  130 
Haldane,  J.  B.  S.,  35,  163,  254 
Haldane's  rule,  288 
haploid,  188 
haploidy,  304f. 


immunology,   11  Of. 
inbreeding,  221,  223 
incompatibility,  255 
independent  assortment,  173,  193 
induced  mutations,  209f.,  353ff. 


416  •  INDEX 


industrial  melanism,  2  5  Off. 

inheritance   of    acquired   characteristics, 

15,  21f. 
Insectivora,  323 
insemination  reaction,  288 
interaction  between  genes,  180ff. 
interference,  198,  314 
intersex,  31  If. 

introgressive  hybridization,  280 
inversion,  201,  229,  255,  257ff.,  314 
ionic  composition,  113ff. 
Iris,  280 
Irish  elk,  43 
Iron  Age,  345,  348 
isoalleles,  179 
isogametes,  306 
isolation,  27 Iff.,  279,  286 
isomorphic,  305 


J 


Java  man,  336 
Jimson  weed,  229 
Johannsen,  W.  L.,  34,  246 
Jones,  D.  F.,  221 
Junonia,  84 


Kant,  Immanuel,  18 

Kinorhyncha,  133 

Klinefelter's  syndrome,  284,  309 


Lagomorpha,  111 

Lamarck,  2 Iff.,  34,  95 

Lamarckianism,  244f. 

Laurasia,  75 

Leakey,  L.  S.  B.,  335 

Leibnitz,  Gottfried  Wilhelm,  18 

lemurs,  324,  330 

leopard  frog,  6ff.,  84,  274,  288 

lethals,  226f.,  246 

leukemia,  358 

life  cycle,  188f. 

Limnopithecus,  332 

Limulus,   111 


linear  order  of  genes,  196ff. 
Lingula,   135 
linkage,  193,  195ff.,  313 
Linnaeus,  19,  81,  83 
liverworts,  I44f.,  151 
lobe-finned  fish,  44,  49 
Lorisiformes,  324f.,  330 
Lucretius,  16 
lung  fish,  44 
Lycopsida,  145,  152 
Lyell,  C,  20,  29,  32 
Lymantria,  312 
Lysenko,  T.  D.,  22 

M 

macroevolution,  294f. 

macula  lutea,  325 

malaria,  257 

male  competition,  316 

male  haploidy,  309 

Malthus,  T.,  19,  27,  360f.,  365f. 

mammals,  46,  323 

man,  12,  92,  101,  323ff. 

marmosets,  327 

marsupials,  46 

Mastigophora,  124 

materialism,  16 

Matthew,  P.,  23 

Matthew,  W.  D.,  75 

Mayr,  E.,  271,  292,  336 

mean,  216 

medicine,  practice  of,  352f. 

megaevolution,  294f. 

Megalopa,  92 

megaspores,  190 

meiosis,  190,  303 f. 

meiotic  drive,  255 

Melandrium,  309 

Mendel,  G.,  33,  164,  l66ff.,  185 

Mendelian  population,  270,  292 

Mesolithic,  345,  348 

Mesozoa,   128 

Mesozoic,  42 

metamorphosis,  10,  91  f.,  121 

metaphase,  186 

Metatheria,  AG 


INDEX  •  417 


Metazoa,  124f.,  128ff. 

Michurinism,  22 

microspores,   189f. 

middle  ear  ossicles,  90 

migration,  237,  278ff. 

mimicry,  llf.,  259,  318 

mink,  coat  color,  180 

mitosis,  185ff.,  303f. 

modern  synthesis,  35 

modifying  factors,  220 

Mollusca,  132,  134ff. 

Mongoloid,  343 

monohybrid,  171 

monosomic,  204 

monotremes,  46 

Moody,  111 

Moore,  J.  A.,  274 

Morgan,  T.  H.,  34 

mosses,  145,  151 

mouse,  brachyury  in,  180 

mule,  155 

Muller,  H.  J.,  35,  254,  289,  353 

multiple  alleles,  177ff. 

multiple  factors,  218ff. 

mutation,  66,  207ff.,  237f.,  247,  350ff.; 

rates  of,  21  Off.,  237f.,  353 
mutation  pressure,  237f. 
mutation  rate  genes,  210 
mutation  theory,  33,  213 
Myxomycophyta,  145,  149 

N 

natural  philosophers,  17f. 

natural   selection,    15,   23,   31,   96,   237, 

239ff.,  266,  316 
natural  system  of  classification,  80 
nature  of  the  universe,  53ff. 
Nauplius,  93 
Neanderthal  man,  336ff. 
Nearctic,  69f. 
Needham,  J.  T.,  59 
Negroid,  343 
Nematoda,  133 
Nematomorpha,  133 
Nemertea,  132 
Neo-Darwinism,  35 


Neolithic,  345,  348 
Ne  optima,  136 
Neotropical,  69f.,  72 
Newton,  Sir  Isaac,  57 
New  World  monkeys,  327 
Nilsson-Ehle,  218 
nitrogen  excretion,  120ff. 
normal  curve,  2l6f. 
notochord,  90,  142 
Nuttall,  G.  H.  F.,  Ill 

O 

oceanic  islands,  75ff. 
Oenothera,  33,  203,  213f. 
Olduvai  Gorge,  333 
Old  World  monkeys,  327ff. 
ontogeny,  87 
Onycophora,   137ff. 
oogenesis,  188 
Oparin,  A.  I.,  62 
orangutan,  327 
Oreopithecus,  335 
organic  compounds,  6 Iff. 
Oriental,  69,  71 
orthogenesis,  43 
orthoselection,  43 
osmosis,  H4ff. 
Osteichthyes,  44 
ostracoderms,  44 
overdominance,  221,  223,  256 
overpopulation,  366ff. 
Owen,  R.,  32 


paedogenesis,  92 
Palearctic,  69f. 
Paleolithic,  345fT. 
Paleozoic,  42 
pangenesis,  33 
Panther  a,  80,  100 
Paracelsus,  57 
paracentric,  201 
Paranthropus,  335 
Parazoa,  128 
parthenogenesis,  314 


418  •  INDEX 


Pasteur,  L.,  59 

Pearson,  K.,  34 

Peking  man,  336 

perfecting  principle,  15 

pericentric,  201 

Peripatus,  138 

Peromyscus,  220,  286 

Phaeophyta,  145,  148 

phenocopy,  244 

phenotype,   172 

phenylthiocarbamide    (PTC),   235 

Philo sophie  Zoologique,  22 

Phoronida,  135 

phosphorylation,  64 

photoreceptors,  106ff. 

photosynthesis,  62,  65f.,  105 

phyletic  evolution,  294 

phylogeny,  83,  87f. 

physiological  isolation,  287f. 

Phytomonadina,  306 

Piltdown  man,  340 

Pithecanthropus,  336fT. 

placenta,  92 

placental  mammals,  46 

Placodermi,  44,  49 

planula  larva,  128,  131 

Platanus,  293 

Platyhelminthes,  129 

platyrrhine,  327 

pleiotropic,  209 

Pliny,  16 

Pliopithecus,  332 

Plunkett,  254 

Pneumococcus,  212,  302 

Pogonophora,  139f. 

pollex,  100 

polygenes,  220,  245 

polymorphism,  81,  249ff.,  270 

Polynesian,  343,  345 

polyploidy,  157f.,  204f.,  229ff.,  281ff. 

polytypic,  81,  250,  270,  34lff. 

Pongidae,  327 

population  structure,  268ff. 

Porifera,  125,   127 

porphyropsin,   108fT. 

position  effect,  203f. 


Potentilla  glandulosa,  228,  272,  274 

preadaptation,  296ff. 

precipitin  test,  111 

prehuman,  332f. 

Priapulida,  133 

Primates,   111,  323ff. 

primitive,  44 

Primula  vulgaris,  255 

Proconsul,  332 

prophase,  186 

Propliopithecus,  332 

prosimians,  323ff. 

Protheria,  46 

Protista,  150 

Protostomia,  132 

Protozoa,  124f.,  150 

pseudoallelism,  203f. 

pseudocoel,  132f. 

Psilophy  tales,  15  If. 

Psilopsida,  145,  152 

pterodactyl,  97 

Pteropsida,  145,  152 

Punnett,  R.  C,  195,  211,  253 

Pyrrophyta,  I45f.,  150 


quantum  evolution,  294ff. 
Quiscalus,  84 

R 

race,  81,  250,  268ff.,  34lff. 

radiation,  209,  350ff. 

Rana  pipiens,  6ff.,  84,  274,  288 

random  mating,  235 

Rapbanobrassica,  205,  220 

Rassenkreis,  84 

rates  of  evolution,  42f. 

Ray,  J.,  19 

rearrangements,   199ff. 

recapitulation,  87,  90,  121,  140 

recessive  traits,  169 

recombination,  177ff.,  301f.,  313ff. 

Redi,  F.,  59 

relict  populations,  73 

Renaissance,  17 


INDEX  •  419 


reproductive  isolation,  286ff. 

reptiles,  45f. 

resistant  strains,  242,  244 

respiration,  65 

reverse  mutations,  237 

Rhodophyta,  l45f. 

rhodopsin,  108ff. 

ribonucleic  acid  (RNA),  150,  l60f.,  301 

Richter,  59 

ring  of  races,  84 

Robinson,  335 

Rotifera,  134 

roundworms,  133 


St.  Hilaire,  2  Iff.,  95 

salivary  gland  chromosomes,  159 

sampling,  263f. 

Santa  Gertrudis  cattle,  241 

Sarcodina,  125,  150 

Scala  naturae,  15 

Schizomycophyta,  145,  148 

schizophrenia,  260 

Scholasticism,  17f. 

seasonal  isolation,  287 

Sedgwick,  A.,  26 

seed  ferns,  152 

segmental  allopolyploid,  205 

segmentation,  100f.,  136f. 

segregation,  I68ff.,  193 

selection  coefficient,   245ff.,   256 

self-duplication,  66,  161 

self-fertilization,  314 

self -sterility,  255,  313 

serial  homology,  100 

sex  chromosomes,   191 

sex  determination,  191,  306ff. 

sex  linkage,  191f. 

sex  reversal,  31  Of. 

sexual  differentiation,  310ff. 

sexual  dimorphism,  316 

sexual  isolation,  287 

sexual  reproduction,  183,  303 

sexual  selection,  239,  298,  315ff. 

sexuality,  303f. 

Shapley,  H.,  66 


sickle  cell  anemia,  257,  342 

Simpson,  34 

Sinanthropus,  336 

Sipunculida,  134,  137 

Sivapithecus,  332 

slime  molds,  145,  149 

Smith,  W.,  20,  23 

snapdragon,   180 

Solarium,  229f. 

Spallanzani,  L.,  59 

Special  Creation,  17 

specialization,  43f. 

speciation,  42,  268,  29lff. 

species,  definitions,  29lff. 

species  concept,  19,  79ff. 

Spencer,  R,  24 

spermatogenesis,  188 

Sphenopsida,  145,  152 

spontaneous  generation,  I4f.,  57,  59 

sporophyte,   189 

Sporozoa,  125 

standard  deviation,  2l6f. 

standard  error  of  the  mean,  217 

standard  error  of  a  ratio,  264 

steady  state  universe,  53,  55 

Strasburger,  E.,  33,  185 

strontium-90,  357f. 

Sturnella,  287 

Suarez,   17 

subspecies,  81,  268ff. 

successive  creation,  22 

Suctoria,  125 

symmetry,  6 

sympatric,  270 

syngamy,  306 

Sy sterna  Naturae,  19,  81 

systematics,  19,  32,  79ff. 

systemic  mutation,  295f. 


tarsiers,  325 
taxonomy,  79ff. 
Tchetverikov,  35 
teleology,  16 
telophase,  186 
terrestrial  life,  117ff. 


420  •  INDEX 


test  cross,  172 

Thales,  57 

thrushes,  80 

T  locus,  255 

tool  tradition,  347f. 

tornaria  larva,  140 

Tracheophyta,  145,  15  Iff. 

Tradescantia,  287 

transduction,  212,  302 

transformation,  160,  212,  302 

transient  polymorphism,  250ff. 

translocation,  202,  229,  314 

tree  shrews,  324 

Trijolium  pratense,  255 

trisomic,  204 

trochophore  larva,  132,  134ff.,  140 

true  fungi,  145,  149 

Turner's  syndrome,  309 

type  concept,  83 


vitalism,  15,  43,  59 
Voltaire,  19 
von  Baer,  K.  E.,  32,  88 
von  Helmholtz,  H.  L.  F.,  59 
von  Tschermak,  E.,  33 

W 

Waddington,  244 
Wallace,  A.  R.,  24,  29f. 
Weismann,  A.,  33,  191 
Wells,  W.,  23 
whales,  111 
wheat  rust,  244 
Wilberforce,  S.,  32 
Wilhelm,   111 
Wolff,  K.  F.,  19 
Wright,  S.,  35,  163,  253 


U 
Ussher,  J.,  51 


Xenophanes,  15,  57 


van  Helmont,  57 

variance,  217 

vascular  plants,  145,  148,  151ff.,  157 

vertebrates,  44f{.,  143 

vestigial  organs,  lOlf. 

viruses,  150,  161,  302 


yellow-green  algae,  l45f. 


Zinjanthropus,  335,  347 
Zoonom'ta,  21 
zygotic,  87  f. 
zygotic  selection,  245 


MARSTON  SCIENCE  LIBRARY 


Date  Due 

Due 

Returned 

Due 

Returned 

|ftV  0  6  19* 

^OCT  24 

1996 

OCT  27  1997 

OCT  2  01997 

UNIVERSITY  OF  FLORIDA 


3    1262    05585    6784 


s*  / 


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